So what are Semantically Related Keywords and should you even care? Oh yes, search engines look for them and they react to them so using them is a SEO must.
These are words which mean the same thing or nearly the same thing. For example:
Now of course they might not mean exactly the same thing in the way that The Big Apple means New York and The Old Smoke means London but search engines like Google understand context.
In a certain context 'Bike' refers to motorbike, in another context it refers to bicycle. If we are creating a page about bicycles and our content is well written we can be confident that 'bikes' will be understood correctly.
If I was writing a piece about bicycles and the only word I ever used when referring to them was 'bicycles' that's actually a fair sign that my content is badly written or spoken if the content is audio or video.
Well written material contains semantically related words and the search engines know it so they expect to find them in high quality content. If they are absent its a down vote for the content.
Remember of course that this is one signal in many but getting all the signals right is what gets you up the rankings so don't ignore this one.
If you have been in the search engine optimization field for some time you might remember keyword density. Advice that if you really wanted Google to think a page was about bicycles then you needed to use the word bicycles between 2 and 4% of the time.
If you used it more than that Google might discount your content because very high keyword density was a sign of text that would be difficult to read. It is a practice known as Keyword Stuffing.
As soon as search engines could process semantically related keywords using something called Latent Semantic Analysis, and that was some years back, pure keyword density as a way to rank went out the window.
Unless you could find a keyword density checker which understood related words using one meant you weren't doing yourself any SEO favors.
Search engines now not only look for related keywords as a way to assess if content is good quality. They look for words that should be in the content if you were talking about a certain subject.
So if I were looking at an article that claimed to be about 'bicycles' I would expect to find:
Not thrown anywhere. Within the content itself in meaningful sentences.
Search engines hold whole pools of words that should be there. The more of them there are the more likely the content is about what it claims to be about.
This process is known as Entity Salience. The entity is the subject and the salience is its importance.
Content can have multiple salient entities. So a piece about bicycle gears has the entity 'bicycles' but it is less salient, less dominant, than the entity 'bicycle gears'.
Now if we're talking about 'bicycle gears' what other words should be there? Not the same as the list for the entity 'bicycles'. There will be some overlap but it is a different pool of words that is expected in good quality content.
Latent Semantic Analysis and Entity Salience make up part of an even larger field Natural Language Processing (NLP) and yes, search engines use this as well.
Here you can start to get really academic by looking at sentence structures and where words sit in relation to each other as ways to determine meanings and entity salience but for SEO work this really isn't necessary because it will happen naturally if you create really useful, high quality content that is easy to read.
Search engines are really just catching up with humans. If you read an article and walk away saying, "Yeah that was a really in depth piece on 'bicycle gears'" then search engines will conclude that as well.
What they can't fully ascertain is whether or not it is a useful piece of content but they have other signals like backlinks which tell them that.
Using semantically related keywords is essential for search engine optimization if you want to signal to search engines that you have rich content. Forget keyword density, that's SEO from yesteryear. You need to be focusing instead on Entity Salience which is a whole lot more than just using the same keyword phrase over and over again.
Entity Salience is more than just using semantically related words, its about making sure you use words and phrases which are related to those words. I tend not to get too hung up about this. If you are writing well for your subject you'll be achieving this. If you are writing 300-500 word articles of verbal vomit full of padding you won't.
I'm Tim Hill, a Search Engine Optimization and Online Marketing specialist. I created this site to help others understand that SEO is not a mysterious black art!.
If your a newbie try the Getting Started in SEO page, otherwise feel free to dig around and learn more.