The lexical meaning of a word has the following components: Denotation(s), shades of meaning, connotations, and stylistic or emotional components. Synonyms are words (or even expressions) than have identical or nearly identical meaning. Very few synonyms are perfectly identical because a language has no use for two words that are exactly the same in every way. Most synonyms have stylistic differences or differ in shades of meaning. Synonyms can be classified depending on which component is non-identical. Ideographic synonyms differ in shades of meaning and are interchangeable, while stylistic synonyms differ in stylistic or emotional components and are not interchangeable.
Ideographic: walking vs. pacing
Stylistic: Homo sapien vs. person
Synonyms can differ in their distribution syntactically, morphologically, or lexically. Lexically, we can say “I won the game”, but not “I gained the game”.
Synonymy differs from hyponymy in that synonymy is a symmetrical relation, while hyponymy is asymmetrical. For example, noun is a synonym of substantive and substantive is a synonym of noun; coffee is a hyponym of drink, but drink is NOT a hyponym of coffee.