What is a word with no synonyms?

Words without synonyms: First to come to mind are the articles, i.e.; a, an, the, that, those, these, my, your, many, any, either, neither, and, or, but (and a number of other words considered to be articles).

What words are used to avoid repetition?

Pronouns are words used in place of other nouns. They reduce repetition, which improves the clarity, pace, and flow of a sentence or paragraph. Without pronouns, sentences would be longer and messier and communication would become more difficult.

What is unnecessary repetition called?

adjective. characterized by verbosity or unnecessary repetition in expressing ideas; prolix: a redundant style.