A backlink is a link from one website to another. If another site links to your page, that link is a backlink for you, and an outbound link for them.
That is the whole definition. What takes explaining is which links actually count, why four different tools will tell you four different numbers for the same site, and why most of the links a site accumulates are worth nothing at all.
What actually counts as a link
Not everything that looks like a link is one as far as a search engine is concerned.
Google is specific about the markup: “Generally, Google can only crawl your link if it’s an <a> HTML element” with an href attribute. Links built in other ways usually are not extracted at all. Links inserted by JavaScript still count, as long as they end up as that same <a href> markup. Source: Google Search Central.
This is why a mention of your brand in plain text is not a backlink, and why a button wired up with JavaScript that never produces an anchor element gives you nothing, no matter how clickable it looks to a person.
Followed, nofollow, ugc and sponsored
A link can carry a rel attribute that tells search engines something about its nature. Google documents three values and is clear about when each applies.
- nofollow. Google says to “use nofollow only when you don’t trust the source”, not as a default on every external link.
- sponsored. For paid placements. In Google’s words, “if you were paid in some way for the link, qualify these links with sponsored or nofollow”.
- ugc. For links created by users, such as in forums or comments, where Google says to “add ugc or nofollow to these links too”.
Source for all three: Google Search Central. A link with none of these attributes is what people call a followed or dofollow link, though dofollow is not a real attribute, just the absence of the others. Whether the qualified ones carry any value is covered in do nofollow links help SEO.
Why backlinks matter
Links do two jobs, and people usually only talk about the second one.
First, they are how pages get found. A crawler moves between pages by following links, so a page nothing links to is hard to discover, which is a common and overlooked reason new pages sit unindexed.
Second, they act as a signal of whether a site is worth ranking. A link is one site spending a little of its own credibility to point at another, and search engines treat that as evidence.
The second job is why links are the hardest part of SEO. Anyone can publish another page, and nobody can publish another site’s decision to link to them.
Links and referring domains are not the same number
This is the distinction that makes most backlink reports misleading, and it is simple once you see it.
A backlink is a single link. A referring domain is a distinct website that links to you at least once. If one site links to you from four hundred pages, that is four hundred backlinks and one referring domain.
The second number is the one that matters. Four hundred links from one blog’s sidebar is one website’s opinion of you, repeated. Forty links from forty different sites is forty opinions, and it is worth far more despite being a tenth of the raw count.
So when a report leads with a large backlink total, look for the referring domain count underneath it. If the gap between them is enormous, the total is being inflated by site-wide links such as footers, sidebars and templates.
Why every tool reports a different number
Run the same domain through Ahrefs, Semrush, Moz and Search Console and you will get four different figures, sometimes wildly different. None of them is lying.
Each third-party tool maintains its own crawler and its own index of the web. They crawl different subsets, at different frequencies, and apply different rules about what to keep, so their counts are estimates of a thing nobody can measure completely.
Search Console is different again. It reports what Google itself has recorded, which makes it the most relevant number for how Google sees you, and it is still not a complete picture of every link that exists.
The practical approach is to pick one tool and watch its trend rather than its absolute figure. A referring domain count rising from 19 to 31 in that tool is real information. The same tool disagreeing with another tool by a factor of ten is normal and tells you nothing about your site.
Be especially careful when a third-party count is enormous and Search Console’s is tiny. That pattern usually means the large number is counting low-value links that Google has either not recorded or does not weigh.
What makes a link worth having
- It comes from a different site. Internal links serve a real purpose, but they are not backlinks and they do not build external credibility.
- The site is relevant to yours. A link from a site in your field carries more meaning than one from an unrelated directory.
- The page it sits on has its own standing. A link from a page nobody reads and nobody links to passes along very little.
- It appears in the body, in context. A link inside a sentence that explains why it is there is a stronger signal than one in a footer template on every page.
- Somebody chose to place it. Links you can add yourself, in profiles, comments and directory listings, are available to everyone, which is precisely why they are worth so little.
The last point is the one that stings, because the easiest links to acquire are the ones with least value, and that ordering is not an accident.
The spam links you never asked for
Most sites eventually pick up links they did not request, from scraper sites, auto-generated directories and link networks. Seeing a few hundred of these appear in a backlink tool is alarming the first time.
Google’s spam policies define link spam as creating links to or from a site “primarily for the purpose of manipulating search rankings”, and state that Google “may act against any type of spam practices we detect”. Source: Google Search Central.
Two things are worth separating here. Links you built or paid for to manipulate rankings are your responsibility. Links that simply arrived, from networks you have never heard of, are a thing that happens to almost every site on the web.
Before acting on either, check Search Console for a manual action, because that is the one unambiguous signal that Google has taken issue with something. Reaching for the disavow tool because a third-party report looks ugly is a decision worth taking slowly, since you are telling Google to ignore links, and that is not reversible in a hurry.
Where to go from here
Understanding what a backlink is leads straight to the harder question of how to earn one. Our guide to starting a link building campaign covers the approaches worth your time, and building blog authority covers the slower route of becoming worth linking to.
If the acronyms in your backlink reports are the confusing part, DA, DR, UR, TF and the rest are all explained in our SEO acronyms guide.
Frequently asked questions
What is a backlink?
A backlink is a link from one website to another, counted from the perspective of the site receiving it. Google generally only treats something as a crawlable link when it is an HTML anchor element with an href attribute, so a plain text mention of your brand is not a backlink. Links do two jobs: they help pages get discovered, and they act as a signal of whether a site is worth ranking.
What is the difference between backlinks and referring domains?
A backlink is a single link, while a referring domain is a distinct website linking to you at least once. If one site links to you from four hundred pages, that is four hundred backlinks and one referring domain. The referring domain count is the more meaningful number, because it counts how many separate sites vouched for you rather than how many times one of them did.
Why do backlink tools show different numbers?
Each tool runs its own crawler and keeps its own index, crawling different parts of the web at different rates with different rules about what to retain. Search Console reports what Google itself has recorded, which is the most relevant view for how Google sees your site. Watch the trend within one tool rather than comparing absolute figures between them.
Do nofollow links count as backlinks?
They are still links, and they still bring visitors and visibility, but the rel attribute tells search engines something about how the link was placed. Google documents nofollow for sources you do not trust, sponsored for paid placements, and ugc for user created links such as forum posts and comments. A link carrying none of these is what people call followed or dofollow, although dofollow is not an actual attribute.
Are spam backlinks harmful?
Most sites accumulate links from scrapers and auto-generated directories without doing anything, and their arrival is not evidence that your site is in trouble. Google’s spam policies target links created primarily to manipulate rankings, which is a different situation from links that simply appeared. Check Search Console for a manual action before acting, because that is the clear signal, rather than reacting to how a third-party report looks.
How many backlinks does a site need?
There is no threshold, because the value depends on which sites are linking rather than how many links there are. A handful of links from relevant, well regarded sites in your field will usually outperform hundreds from unrelated directories. Track referring domains over time instead of chasing a total.