Although Continuous Integration is a practice that requires no particular tooling to deploy, we’ve found that it is useful to use a Continuous Integration server. – Martin Fowler

The map is not the territory. The tool is not the practice. Having a Continuous Integration Server is not the same as continuously integrating. Continuously integrating means that your team is:-

• working on the same source control branch
• checking in and pushing frequently, ie several times a day
• only pushing working code that keeps the test clean
• avoiding long-lived feature branches

A CI server is a useful but not essential tool for continuous integration; there are teams running a CI server that are not continuously integrating.