What is the definition of mean median and mode for a measure of center?

The mean is the same thing as the average. To find it you add all the numbers together and then divide by the number of numbers you added.

The median is the one that is in the middle. To find it you put all of the numbers in descending order and cross off from each end until you reach the middle and then that's the median.

The mode is the number that occurs the most frequently.