Stocks that have been going up recently tend to keep going up; the ones falling tend to keep falling — for medium horizons.
Jegadeesh-Titman 1993: 12-month-minus-1-month price return is one of the most replicated anomalies in finance. We use 12m−1m return + revenue acceleration. The "minus 1m" part avoids the well-known short-term reversal effect (Jegadeesh 1990). High momentum = recent winner. Low momentum = recent loser.