significance level

Short definition from Wiktionary

Noun

1. 1
   A measure of how likely it is to draw a false conclusion in a statistical test, when the results are really just random variations.
2. 2
   The probability, usually expressed as a percentage, of making a decision to reject the null hypothesis when the null hypothesis is actually true; the probability of making a type I error.
3. 3
   Used other than figuratively or idiomatically: see significance, level.