The term confusables includes words with similar but non-identical meanings like imply/infer and less/fewer. A term that applies principally to similar or related meanings is
(linguistics) A term whose meaning is similar, but not identical, to
that of another term. Wiktionary
A word that has almost the same meaning as another word:
"hungry" and "peckish" are near-synonyms.
Near synonyms are more
common than perfect synonyms.
Near-synonyms may be
regarded as equivalent for some purposes, but not others. Cambridge
case. This multifaceted word is often a sign of verbal inflation, especially in its uses as a near-synonym of situation.
include; comprise. The basic difference between these near-synonyms is that include implies nonexclusivity..., while comprise implies exclusivity... Garner; The Chicago Guide to Grammar, Usage and Punctuation
A list of commonly used near-synonyms
Here are some examples of commonly used near-synonyms for legal
concepts used in legal documents. There are subtle differences in
meaning or usage between them.
... Rupert Haigh; Legal English p.81
Two or more words or ideas that can easily be confused
confusables such as ‘principle’ and ‘principal’ Oxford Advanced Learner's
(The above definition of the singular as "two or more ..." bothers me.)
A word or phrase that is easily confused with another in meaning or
usage such as mitigate which is often confused with militate. New
Oxford American Dictionary
A word or phrase that is easily confused with another Wiktionary
(I do not see the noun in the OED.)
A related term is partial synonym
See What is the difference between partial and total synonyms?
Synonymy, as we have seen, is defined where two lexemes are mutually
hyponymous, in other words where their extensions (and senses) are
required to be identical, as shown in (61)
definition does not allow for partial synonymy, since extensions
in the theory are discrete, i.e. there are no fuzzy edges where, for
example there may be things that are almost pullovers but that are not
a sweater. Since, as we have noted, full synonymy is rarely, if ever,
attested, this makes the definition in (61) less useful than it might
be. Ronnie Cann; Formal Semantics, p. 220