A trend is the persistent directional movement of an asset's price over a given time frame. An uptrend is defined by a sequence of higher highs and higher lows; a downtrend by lower highs and lower lows; and a sideways or ranging market by roughly equal highs and lows with no clear direction. Trend identification is the first step in most technical and timing frameworks.
The practical challenge is that trends exist at multiple timeframes simultaneously. A stock can be in a short-term downtrend (falling over the last three weeks) within a medium-term uptrend (rising over the last three months) within a long-term secular bull market. The timeframe relevant to a timing decision depends entirely on the holding period and objective of the strategy.
Trend-following strategies — which buy when a trend is established and sell when it ends — are among the oldest and most studied timing approaches. They tend to perform well in strongly trending markets and poorly in choppy, range-bound conditions. The core assumption (that trends persist long enough to exploit once identified) is empirically supported over long histories but fails in specific regimes, which is why no trend strategy produces consistent results across all market environments.