概率，又称或然率、机会率或几率、可能性，是数学概率论的基本概念，是一个在0到1之间的实数，是对随机事件发生之可能性的度量。

概率常用来量化对于某些不确定命题的想法，命题一般会是以下的形式：「某个特定事件会发生吗？」，对应的想法则是：「我们可以多确定这个事件会发生？」。确定的程度可以用0到1之间的数值来表示（0表示不可能发生，1表示一定会发生），这个数值就是机率。因此若事件发生的机率越高，表示我们越认为这个事件可能发生。像丢铜板就是一个简单的例子，正面朝上及背面朝上的两种结果看来机率相同，每个的机率都是1/2，也就是正面朝上及背面朝上的机率各有50%。

Probability is the measure of the likelihood that an event will occur. Probability is quantified as a number between 0 and 1 (where 0 indicates impossibility and 1 indicates certainty). The higher the probability of an event, the more certain we are that the event will occur. A simple example is the tossing of a fair (unbiased) coin. Since the coin is unbiased, the two outcomes ("head" and "tail") are equally probable; the probability of "head" equals the probability of "tail." Since no other outcome is possible, the probability is 1/2 (or 50%) of either "head" or "tail". In other words, the probability of "head" is 1 out of 2 outcomes and the probability of "tail" is also, 1 out of 2 outcomes.