A punctuation mark (') used to show possession or missing letters, or a figure of speech where a speaker addresses someone absent or something non-human.
From Greek 'apostrophe' meaning 'a turning away,' from 'apo-' (away) and 'strophe' (turning). The punctuation mark came later, named for the 'turning away' from regular text flow when showing a contraction.
Shakespeare was a master of the rhetorical apostrophe—'O Romeo, Romeo, wherefore art thou Romeo?' literally addresses an absent person—and it's completely different from the tiny punctuation mark that causes so much spelling confusion!
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