The statement that I want to make is:
"Impostor syndrome" ... why does this (conjunction of words) sneak into every text I read these days?
What is the right or best expression for "conjunction of words"? In the statement I don't want to refer to the syndrome itself, but rather to the expression. If it was one word instead of two I would want to say
... why does this word sneak into every text I read these days.
________ why do these two words sneak into every text I read these days.
would miss the point, because it's not these two words that sneak into every text at various location but rather their conjunction.