Continuous Integration - aka CI - is the practice of often / regular code merges into central (mainline) branch in central repository.
Regular means at least daily. Often hourly. Or more frequent. It also means every member in the developer teams release at that frequency. In that sense, it is also a workflow - or at least requires a compatible workflow like Trunk Based Development.
The goal is to minimize merging issues, that would result from developers working on the same code base, working in different (feature) branches over a (too) long time.
CI is a preceding or a the first stage / step in a Deploy Pipeline.
- Wikipedia: Continuous integration
- Martin Fowler: Continuous Integration
- Atlassian: What is Continuous Integration?
- AWS: What is Continuous Integration?