eCAP is a software interface that allows a network application, such as an HTTP proxy or an ICAP server, to outsource content analysis and adaptation to a loadable module. For each applicable protocol message being processed, an eCAP-enabled host application supplies the message details to the adaptation module and gets back an adapted message, a "not interested" response, or a "block this message now!" instruction. These exchanges often include message bodies.
The adaptation module can also exchange meta-information with the host application to supply additional details such as configuration options, a reason behind the decision to ignore a message, or a detected virus name.
If you are familiar with the ICAP protocol (RFC 3507), then you may think of eCAP as an "embedded ICAP", where network interactions with an ICAP server are replaced with function calls to an adaptation module.
This site documents eCAP interfaces and provides open source software required for their implementation.