The urge to enter a trade because price is moving without you, usually late, without a plan, and with a poor risk-reward.
The mood around a market cycle. The same price path labelled with the feelings that tend to travel with it, from optimism up to euphoria and down through panic to despondency. Confidence is highest where the most money is already committed and prices are highest.
FOMO entries happen after the move, near the point where early buyers are selling. They typically have no defined stop-loss because there was no setup, and they are the raw material of bull-traps and blow-off-tops.
The antidote is accepting that missed trades cost nothing, while chased trades cost real money.
Example: a stock runs 30% in an hour on news. A trader buys at the high because it keeps going. It closes 15% below her entry.