Relevant to whom? To you? To the Search Engine? To the user?
Mmmm... I shall attempt an answer to this and to peter's post:
(Note: The following example is dedicated to the honourable moderator qwerty)
For reasons best known to himself, qwerty has decided to never again us the word 'wi*get' in an SEO example, but rather to use, among other things, 'lederhosen'. This choice has been endorsed by dragonlady and torka and others. So who am I to argue? Anyway, back to your question...
So qwerty launches a website about lederhosen. Particularly 'red lederhosen', which are apparently a favourite of his. And it's the best darn site on the www about red lederhosen, packed full of content.
But, being a trifle naive about matters SEO, he doesn't actually use the keyphrase 'red lederhosen' anywhere on his website. He talks about 'crimson leather apparel' and 'Austrian-style scarlet shorts', but never does he mention the commonly used term 'red lederhosen'.
So, when dragonlady and torka search the www on google for 'red lederhosen', they don't find qwerty's world-beating site, but a load of other inferior sites that happen to use the magic term 'red lederhosen' in their content. And those sites are so inferior that dragonlady and torka are totally disenchanted, give up their search and, what's more, vow to use ATW in future, because they blame google for having a poor algo.
Then along comes some smart-arse SEOer who persuades qwerty that most people are searching on the term 'red lederhosen' and he should optimise his most excellent site accordingly. And so it comes to pass.
The site duly goes to #1 in the rankings, and both the users and google are well-served by this site being optimised for 'red lederhosen'.
Can we discuss something else now please!? <g>