WPM is an abbreviation of Words-Per-Minute. Confusingly this does not literally mean whole words, as one might find in a dictionary, but rather word-units. For speed to be comparable, it must be measured in standard units. In the case of typing speed if we used actual words for the WPM measurement then typing speed test results would not be comparable unless everyone used the same texts for their respective typing speed tests - which would give us the additional factor of memorisation so, the word-units we use are artificial.