What is Continuous Integration (CI)?
Continuous integration means every developer merges small changes into the shared mainline at least daily, and every merge triggers an automated build and test run. Broken builds get fixed immediately — integration pain shrinks from a dreaded phase to a background hum.
Its extensions: continuous delivery (every green build is deployable) and continuous deployment (every green build deploys). The through-line is batch size: small integrations fail small.
Worked example
Before CI, a bank's release team spent two weeks each quarter merging four teams' branches — "integration hell," scheduled. After: merges land daily, 4,000 tests run per commit, and a Tuesday-morning breakage is fixed by 10am by the person who caused it at 9:40. The quarterly merge phase didn't get faster; it stopped existing.