fin

From Wiktionary

Pronunciation[change]

Noun[change]

Singular
fin

Plural
fins

Dolphins have fins
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The Simple English Wikipedia has an article on:
  1. (countable) A fin is a wing like organ that is attached to some parts of the body of undersea animals. It is used for turning in a certain direction, balancing, and for propelling themselves.
    Some fish have a dorsal fin.