Assuming you are referring to the normal distribution, and depending on the context, it could be sample value, observed value, sample limit, etc.
In the z transformation formula, the x stands for the raw score or observation from the dataset.
In the z transformation formula, the variable x represents an individual data point in a dataset. It is the value that we want to transform into a standardized score using the z-score formula. To calculate the z-score, we subtract the population mean (M) from the individual data point (x) and divide it by the standard deviation (s) of the population. The resulting z-score tells us how many standard deviations an individual data point is away from the mean.