The status code a server returns when a page does not exist. If a page that was indexed starts returning 404 (or 410, gone), Google drops it from the index. A few 404s are normal on any site.
Example
Meera removes a discontinued kurta and its URL now returns 404. That is fine. But a popular page that other sites link to should be redirected to a close replacement instead.
How you'll hear it
“These 404s are old junk URLs; ignore them. That one, though, has backlinks; redirect it.”
Don't confuse it with
- Soft 404: a page that says 'not found' or is empty but returns a success code. Google flags these separately because the code is misleading.
- 301 redirect: sends the old URL somewhere useful.
Related terms
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Sources
- How HTTP status codes, and network and DNS errors affect Google Search, Google Search Central