The text body that accompanies a social-media post — on Instagram and LinkedIn, often the difference between scroll-past and engagement.
Captions are the text alongside an image or video post. On Instagram, captions can be up to 2200 characters, but the first 125 characters (the "above the fold" preview) determine whether viewers tap "more." On LinkedIn, the first 3 lines preview before the "see more" cutoff. On X, the entire post IS the caption — there's no separation.
Effective captions: open with a hook (not a greeting), use line breaks every 1–2 sentences for mobile readability, end with a single specific CTA, optionally include 3–5 hashtags below the text or in the first comment.
A separate use of "caption" refers to the on-screen text burned into video — the words that appear synced to the speaker. Those are also called subtitles or closed captions. Both meanings show up in content-creator vocabulary and context disambiguates which is meant.