A person or thing that appears; someone who makes an appearance, especially in a public or official context.
From 'appear' (to come into view or become visible) plus the agent suffix '-er,' creating a noun for whatever performs the action of appearing. This is a rare but grammatically valid formation.
Most of us wouldn't use 'appearer' because we'd just say 'someone who appears' or 'the person appearing,' but technically it's a perfectly valid English word. It shows how not all possible words become popular even when they're grammatically perfect.
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