Trading Signal
A precisely defined condition in market data that tells a strategy to act — enter, exit, or change position size.
A trading signal is the trigger in a strategy: a condition that is either true or false on a given bar, such as "close above the 20-day high" or "RSI below 30 while price is above the 200-day moving average". When it fires, the rules say what to do.
A usable signal is unambiguous, computable from data available at the time (no look-ahead bias), and specific about when the order is placed — usually the next bar after the signal bar closes. Signals can be binary (buy / don't buy) or continuous (a score that scales position size).
A signal alone is not a strategy. It still needs exits, sizing and costs before its expectancy means anything, and a signal that looks predictive on its own can lose money once slippage and commissions are included.
On the desk
Signal: the 50-day moving average closes above the 200-day (a golden cross). It fires on Tuesday's close; the strategy buys at Wednesday's open. Over 20 years on one index it fires 11 times — too few trades to judge the signal on that market alone.