Cacheable

A cacheable response is an HTTP response that can be cached, that is stored to be retrieved and used later, saving a new request to the server. Not all HTTP responses can be cached, these are the following constraints for an HTTP response to be cached:

The method used in the request is itself cacheable, that is either a GET or a HEAD method. A response to a POST or PATCH request can also be cached if freshness is indicated and the Content-Location header is set, but this is rarely implemented. For example, Firefox does not support it (Firefox bug 109553). Other methods, like PUT or DELETE are not cacheable and their result cannot be cached.
The status code of the response is known by the application caching, and it is considered cacheable. The following status code are cacheable: 200, 203, 204, 206, 300, 301, 404, 405, 410, 414, and 501.
There are specific headers in the response, like Cache-Control, that prevents caching.

Note that some non-cacheable requests/responses to a specific URI may invalidate previously cached responses on the same URI. For example, a PUT to pageX.html will invalidate all cached GET or HEAD requests to the same URI.

When both, the method of the request and the status of the response, are cacheable, the response to the request can be cached:

GET /pageX.html HTTP/1.1
(…)

200 OK
(…)

A PUT request cannot be cached. Moreover, it invalidates cached data for request to the same URI done via HEAD or GET:

PUT /pageX.html HTTP/1.1
(…)

200 OK
(…)

A specific Cache-Control header in the response can prevent caching:

GET /pageX.html HTTP/1.1
(…)

200 OK
Cache-Control: no-cache
(…)