A rotation turns every point of a shape around a fixed centre point by a given angle. The size and shape stay the same, just the orientation changes.
Rotating 90° clockwise around the origin: (x, y) → (y, −x). Rotating 180°: (x, y) → (−x, −y). Rotating 270° clockwise (= 90° counter-clockwise): (x, y) → (−y, x).
Translations, reflections, and rotations are all rigid transformations — they preserve size and shape perfectly.
Remember
180° rotation always flips both signs: (x, y) → (−x, −y). It's the simplest one to remember.
Real World
The hands of a clock rotate around the centre. The minute hand makes a full 360° rotation every hour.
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