Either (1) a specific server environment, or (2) the process of combining disparate systems at the code level.
As a server environment, integration is meant as a location for where code from multiple developers is combined and tested.
Sometimes synonymous with a testing environment, though integration servers tend to see a lot of new code. Every time a developer submits code to the the source code management system, that code will likely be deployed to integration. This code will be raw and untested for even basic execution. As such, there might be a separate testing server to which more mature code is deployed less frequently.
As a verb, to “integrate” software is to make disparate systems work together at some level. For instance, the process of allowing your CMS to push data into your new release management system would be to “integrate” the two systems. Integrations are often packaged and sold as connectors.
Finally, a website itself is occasionally referred to as an “integration” of a CMS, rather than an “implementation,” though this is not common.