A synonym is a word having the same or nearly the same meaning as another word in certain contexts. The adjective form is synonymous. Synonymy is the relationship that exists between words with closely related meanings. The word comes from the Greek meaning "same name." Contrast with an antonym. A synonym for the term synonym is poecilonym.

Synonyms occur in a language in different contexts, such as formal and informal language, like you'd use in conversation vs. a business or academic paper. Also, some synonyms have slightly different connotations when they're used, even though they might mean the same thing. For example, look at the differences between the terms for money: moolah, greenbacks, cash, currency, and revenues, which all occur in different contexts and levels of formality. Synonyms may also be nested inside one another, which is referred to as a kangaroo word.

Also, English has inherited and borrowed many words from other languages and kept some duplicates. (It's also why some words have sound alike or don't spell out as they sound, but those are additional topics.)

Synonyms saw a rise in the English language during the Norman conquest when the ruling class spoke Norman French and the lower classes continued to speak Old English. As a result, the Norman-originated word people exists alongside the Saxon-derived folk.

There is some debate as to whether two words can truly be synonymous. If they're different words, they must mean something slightly different or have contexts where you'd use one or the other, the reasoning goes, which makes them only nearly synonymous but not truly the same thing.

Two words just can't be completely interchangeable in all occurrences. When would two words have all of exactly the same meanings?

When a language does have two different words that mean the exact same thing, oftentimes one will stop being used, as it's unnecessary, or it will take on a different meaning over time. And two synonyms used in different contexts, by definition, can't be exactly the same.

At best, the theory relates, that the closest absolute synonymity can get is a technical word for something and a common name used in conversation for the same thing or dialect differences, such as between British English and American English (lorry vs. truck, boot vs. trunk.)

However, if we look at the definition of synonymous, that is, words meaning nearly the same thing rather than exactly the same thing in every situation, the theory that synonymity is impossible might just not hold up.

This aspect of English—having words that have different meanings in different contexts, as well as having doublets and triplets—is also a result of the language having inherited and borrowed so many words from different languages.

The doublets frail and fragile came to English from the same Latin root fragilis, but one came from French and one from Latin directly. The triplets real, royal, and regal came from Anglo-Norman, French, and Latin, notes Britannica.

Near-synonyms would be just that—words that are closely related as to just about be interchangeable but have different connotations, attitudes, or implications that you'd want to be aware of, making one word more appropriate for a context than another. Anything you can find in a thesaurus has a list of near-synonyms galore. For example, lie finds falsehood, fib, misrepresentation, and untruth, each with different nuances and shades of meaning that each can give to the context it's used in.

It can be tricky when translating between languages, as you need to know the implications and connotations for the word in the original language and make sure you pick up those nuances in the destination language. 

Paul Dickson's book "Intoxerated" claims, "The English language includes more synonyms for 'drunk' than for any other word." Here are just a few of the 2,964 synonyms for drunk in his book: