Test pyramid
The test pyramid is the testing-strategy heuristic that recommends many fast unit tests at the base, fewer integration tests in the middle, and very few slow end-to-end tests at the top. The pyramid shape reflects the cost/value ratio: unit tests catch defects cheaply; E2E tests catch a small but irreplaceable class of integration defects expensively.
The pyramid was popularised by Mike Cohn around 2009 and remains the dominant heuristic for test-suite composition. The inversion (the 'ice-cream cone' anti-pattern) is too few unit tests, too many E2E tests, typically 10x more expensive to run and maintain than a healthy pyramid. The proportions vary by domain but a rough target is 70% unit / 20% integration / 10% E2E by count, with the run-time roughly inverted (E2E may be slow but few; unit may be many but fast). Modern variants (the 'testing trophy' from Kent C. Dodds, emphasising integration tests for frontend code) adjust the proportions without rejecting the pyramid's core logic.