Definition of 'Normal Distribution'
A probability distribution that plots all of its values in a symmetrical fashion and most of the results are situated around the probability's mean. Values are equally likely to plot either above or below the mean. Grouping takes place at values that are close to the mean and then tails off symmetrically away from the mean.
Also known as a "Gaussian distribution" or "bell curve".
Investopedia explains 'Normal Distribution'
The normal distribution is the most common type of distribution, and is often found in stock market analysis. Given enough observations within a sample size, it is reasonable to make the assumption that returns follow a normally distributed pattern, but this assumption can be disproved.
As with any distribution, the distributions mean, skewness and kurtosis coefficients should be calculated in order to determine the type of distribution you may be dealing with.