What is Z-score (standard score)?
Also known as: z-value
A number that shows how many standard deviations a single observation lies from the mean. Formula: z = (x − mean) ÷ standard deviation. A positive z value means above the mean, a negative z means below it, and zero means exactly at the mean. The main benefit of the z-score: it brings indicators in different units of measurement or at different levels onto a single scale and makes it possible to compare them directly. Converting a z-score into a probability (for example, "in what percentage of cases such a result occurs") is appropriate when the data are assumed to be approximately normally distributed.
Source: Online Statistics Education (Rice University): z score