Google Ranking Factor – Keywords in Title Element
The Page Title element in the <head> of the HTML document should contain at least one mention of the primary keyword phrase being optimised, and should bear the greatest relevance to the thematic content of the page.
Learn more about the page title element here.
The keyword phrase can have some minor morphological differences – the shape of the word – to the primary keyword phrase, but ideally should be a morphological match – the same or similar shape. In other words: you could write “plumber” instead of “plumbing” and it would still be considered to be a keyword match.
Google knows how words are related.
The keyword phrase may also have a semantic match without a morphological one.
In this case, you might write “company” instead of “firm” or “business”. Those don’t match by shape at all, but they match closely by meaning.
Both semantic and morphological matches are considered, and as the algorithm learns over time it’s ability to interpret meaning is now truly incredible thanks to RankBrain, Google’s AI software.
Here’s an interesting article by Kristine Schachinger writing for Search Engine Journal discussing RankBrain.