Volume
Definition
The number of shares or contracts traded in a security during a given period.
Detailed Explanation
Volume measures trading activity - how many shares change hands. It's crucial because it validates price movements. Strong moves on high volume are more likely to continue than moves on low volume. Breakouts should occur on above-average volume to be considered valid. Volume divergence (price making new highs but volume declining) often precedes reversals. 'Volume precedes price' is a trading axiom.
Example
GOOGL breaks above resistance on 3x average volume = high-conviction breakout. Same break on 0.5x average volume = suspect, likely to fail.