Integration in software engineering means combining different code changes from multiple developers into a single, working project. It's the critical moment when individual features—like a login screen or database—merge to function as a whole, verifying they don't break existing functionality.

Previously, teams used a 'Merge Day' approach under a traditional/waterfall model. Developers worked in isolation for weeks, then attempted a massive code merge. This often led to Integration Hell: massive merge conflicts, delayed bug discovery, and the 'It Works on My Machine' syndrome, causing perpetual delays.

Continuous Integration (CI) solves this by making integration a daily habit. Developers merge small changes frequently, and automated systems like Jenkins or CodeBuild immediately build and test the code. This catches errors instantly, replacing manual 'Merge Days' with automated pipelines that ensure a single source of truth.