This is also called a simple average and an arithmetic average.
It is calculated by adding up the items to be averaged and then dividing by the total number of items. So for example if a share closes over five days at £2, £2.20, £2.80, £2.40 and £2.60 the mean is ((£2+£2.20+£2.80+£2.40+£2.60)/5), or £2.40. Although commonly used to measure an average, the mean can be skewed by outliers – items with very low or high values – particularly where the sample size is small. It also sometimes gives odd results. For example the mean number of children being born in Spain is around 1.7!