What is a character limit?
A character limit is the maximum number of characters a platform allows in a post, caption, bio or comment; text over the limit is cut off or can't be posted.
Why it matters
Limits differ widely between platforms, and some cut off the visible preview long before the hard limit, so the first line of a caption matters most.
Example
A standard post on X holds up to 280 characters, while an Instagram caption can hold up to 2,200.