Apache Reference: mod_rewrite, RewriteEngine

RewriteEngine

Rewrite Engine Operation Switch
Syntax: RewriteEngine on|off
Example: RewriteEngine on
Since: Apache 1.2

This directive enables or disables the runtime rewriting engine. If it is set to off, then mod_rewrite does no runtime processing. Use this directive to disable mod_rewrite instead of commenting out all Rewrite==Rule directives. By default, rewrite configurations are not inherited. Thus you need a RewriteEngine directive to switch this configuration on for each virtual host in which you wish to use it.