Google was no help. It found many synonyms for almost, nearly, close, etc. Is there a noun that means "not quite a synonym"?
car : wagon
hide : lose
parasynonym (plural parasynonyms)
(linguistics) A word or phrase that shares similar meanings with another term in some > contexts, but not all; a close synonym.
Similar means "resembling without being identical", but I imagine you're looking for something fancier.
Some dictionaries think synonymous itself will work:
adjective: synonymous (of a word or phrase) having the same or nearly the same meaning as another word or phrase in the same language
Do you mean something like the word like or alike?