標題の2つの用語は言語学ではいずれもよく使われるが，はたして同じものなのか，それとも異なるのか．語の意味にまつわる話題であり，案外難しい問題である．実用上は，厳密に定義を与えないままに，両用語を「適当に、常識的に」使用し（分け）ているものと思われる．実際，英語の synonym は「同義語」にも「類義語」にも相当するのだ．
　一般的には，語義が完全に一致する「完全同義語」 (absolute synonym) のペアは（ほとんど）ないと言われる (cf. 「#1498. taboo が言語学的な話題となる理由 (3)」 ([2013-06-03-1])) ．ということは，synonym という用語が有意味となるためには，「完全同義語」ではなく「不完全同義語」を，すなわち「類義語」ほどを指していることが望ましい．一方，英語には near-synonym あるいはより専門的に plesionym (< Gr. plēsios （近接した） + onoma （名前）) という用語もある．ここから，synonym, near-synonym, plesionym という用語は，それこそ互いに synonymous な関係にあると解釈できそうである．語義の似ている度合いというのは数値化できるわけでもなく，当然ながら微妙な問題になることはやむを得ない．
　Cruse (176--77) の意味論の用語辞典の説明を覗いてみよう．

synonymy, synonyms A word is said to be a synonym of another word in the same language if one or more of its senses bears a sufficiently close similarity to one or more of the senses of the other word. It should be noted that complete identity of meaning (absolute synonymy) is very rarely, if ever, encountered. Words would be absolute synonyms if there were no contexts in which substituting one for the other had any semantic effect. However, given that a basic function of words is to be semantically distinctive, it is not surprising that such identical pairs are rare. That being so, the problem of characterising synonymy is one of specifying what kind and degree of semantic difference is permitted. One possibility is to define synonymy as 'propositional synonymy': two words A and B are synonyms if substituting either one for the other in an utterance has no effect on the propositional meaning (i.e. truth conditions) of the utterance. This is the case with, for instance, begin: commence and false: untrue (on the relevant readings):

   The concert began/commenced with Beethoven's Egmont Overture.
   What he told me was false/untrue.

By this definition, synonyms will typically differ in respect of non-propositional aspects of meaning, such as expressive meaning and evoked meaning. Thus, begin and commence differ in register; the difference between false and untrue (indicating lack of veracity) is rather subtle, but the former is more condemnatory, perhaps because of a stronger presumption of deliberateness. However, while this is a convenient and easily applied way of defining synonymy, it does not capture the way the notion is used by, for instance, lexicographers, in the compilation of dictionaries of synonyms or in the assembly of groups of words for information on 'synonym discrimination'. Certainly, some of the words in such lists are propositional synonyms, but others are not, and for these we need some such notion as 'near-synonymy' ('plesionymy'). This is not easy to define, but roughly speaking, near-synonyms must share the same core meaning and must not have the primary function of contrasting with one another in their most typical contexts. (For instance, collie and spaniel share much of their meaning, but they contrast in their most typical contexts.) Examples of near-synonyms are: murder: execute: assassinate; withhold: detain; joyful: cheerful; heighten: enhance; injure: damage; idle: inert: passive.

　synonyms を分析するにあたって，命題的意味 (propositional meaning) と非命題的意味 (non-propositional meaning) を区別するという方法は確かにわかりやすい．これは概念的意味 (conceptual meaning) と非概念的意味 (non-conceptual meaning) の区別にも近いだろう（「#1931. 非概念的意味」 ([2014-08-10-1]) を参照）．命題的意味や概念的意味はイコールだが，非命題的意味や非概念的意味が何かしら異なっている場合に，その2語を synonyms と呼ぼう，というわけだ．
　しかし，類義語辞典などに挙げられている類義語リストを構成する各単語は，たいていさらに緩い意味で「似ている」単語群にすぎない．最も中心的な意味のみを共有しており，かつ互いに異なる周辺的な意味はあくまで2次的なものとして，当面はうっちゃっておけるほどの関係であれば (near-)synonyms と呼べるということになろうか．ややこしい問題ではある．

　・ Cruse, Alan. A Glossary of Semantics and Pragmatics. Edinburgh: Edinburgh UP, 2006.

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