Apache Reference: mod_expires

mod_expires - Creation of HTTP Expires Header
Since Apache 1.2, src/modules/standard/mod_expires.c
Andrew Wilson (1996)

mod_expires controls the setting of the HTTP Expires header field in server responses. The expiration date can be set relative to either the time of the source document's last modification, or the time of the last client access. This information informs the client and intermediate HTTP proxies about the document's validity and persistence. If cached, the document may be fetched from the cache rather than from the source until the expiration date has passed. After that time, the cache copy is considered ``expired'' and invalid, and a new copy must be obtained from the source.

Directives:

ExpiresActive	ExpiresByType
ExpiresDefault