Hysteresis loop
The curve showing how a ferromagnet's magnetisation lags the applied field — and why a written bit stays put.
Plot magnetisation (B or M) against the applied field (H) and sweep the field back and forth, and the material doesn't retrace its path: it traces a closed loop. The loop's corners are the material's three key numbers — saturation (Ms), remanence at H=0, and coercivity, the reverse field needed to zero the magnetisation. The area of the loop is the energy lost per cycle.
For storage you want an almost rectangular loop: the medium should sit firmly in one of two states and flip sharply as the write field crosses the coercivity. The term was coined by the Scottish physicist James Alfred Ewing in the 1880s, from the Greek hysteros, "to lag behind".