Buying what has already been rising, on the observation that trends tend to persist for a while before they reverse. It points in the opposite direction to value investing: a stock that has doubled is more expensive against the same earnings, and momentum buys it anyway. The difficulty is exits, because the same evidence that a trend persists also says it ends, usually sharply. PSEye's movers page ranks real returns over one week to two years, and RSI on the analytics page measures how stretched a recent run is.
See also
See every term in the full PSE investing glossary.