An initialism is a new word created from the first letters of a phrase, but read letter-by-letter.
An acronym is a new word created from the first letters of a phrase, and read like a complete word.
An abbreviation is a shortened form of a word.
Idioms are the common phrases native speakers learn as they grow up, but are hard for non-native speakers to learn. Idioms (A) Idioms (N) Idioms (B) Idioms (O) Idioms (C)…
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Note that many synonyms do not have exactly the same meaning; take the time to learn the differences.