A small computer program designed to be add capabilities to a larger piece of server software.
Common examples are "Java servlets", which are small programs written in the Java language and which are added to a web server. Typically a web server that uses Java servlets will have many of them, each one designed to handle a very specific situation, for example one servlet will handle adding items to a "shopping cart", while a different servlet will handle deleting items from the "shopping cart."