Code Coverage
Code coverage measures the percentage of source code — lines, branches, or functions — that gets executed when a test suite runs, giving a quantitative signal for how much of the codebase tests actually touch. It's a coverage-of-execution metric, not a measure of whether that execution actually verified anything.
Several coverage flavors measure slightly different things: line coverage (was this line executed), branch coverage (was each conditional path taken, both true and false), and function coverage (was this function called at all) — branch coverage is generally considered the more meaningful of the common metrics, since it accounts for conditional logic line coverage alone can miss.
The well-known trap is treating a high coverage percentage as proof of quality — a test can execute a line without meaningfully asserting anything about its output, hitting 100% coverage while verifying almost nothing. Mutation testing exists specifically to expose that gap between "code ran" and "code was actually checked."
