Continuous Integration. A good example of that service would be something like http://travis-ci.org or https://jenkins-ci.org. Basically it's a service that runs some predefined task (usually your test suites) at important points: when something is pushed to or merged into master, when a Pull Request is opened, etc...

It also, um, builds your code. Some of us still use compiled languages :P

It stands for continuous integration.

Continuous Integration (testing)