The tendency of humans to act collectively in the face of danger or opportunity—panic-selling when a market falls sharply and rushing to buy when a market rallies. Because this behaviour is predictable, it is the foundation of every professional operator's edge: shake-outs, up-thrusts, and buying climaxes all depend on the herd reacting as expected. The professional trader isolates himself from the herd and becomes a predator rather than a victim, buying when the herd panics on bad news and selling when the herd buys on good news.