A bimodal distribution is a probability distribution with two different modes. These appear as distinct peaks (local maxima) in the probability density function for continuous distributions and the probability mass function for discrete distributions..

A bimodal distribution can arise as a mixture of two different unimodal distributions (i.e. distributions having only one mode). In other words, the bimodally distributed random variable X is defined as $Y$ with probability $\alpha$ or $Z$ with probability $(1-\alpha)$, where $Y$ and $Z$ are unimodal random variables and $0 <\alpha <1$ is a mixture coefficient.

Below is an example of a bimodal distribution:

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When the two modes are unequal the larger mode is known as the major mode and the other as the minor mode.

-- Wikipedia

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