In probability theory, possible deviations from the expected normal dispersion of a random value is described as variance. The variance of a random value is an measure of how far a value can deviate in the short-term from the expected long-term result or mid-value.
In a poker context, which has a random element, variance describes the possible short-term deviation from the long-term expected value of a player. For example, a player in Texas Hold'em with a flush draw on the flop gets his flush on the river around 36% of the time in the long-term. Over just a few hands it may also be the case that he only hits a flush 25% of the time i.e. that he misses his draw five times in a row.
The results of a consistently good player compared to a consistently bad one only rarely indicates that the good player takes winnings in every poker round. Somethimes the bad player simply gets lucky and variance is on his side.
Upswing, Downswing, Tilt