a number of occurrences (hits) of an item normalised per million, also called as i.p.m. (instances per million). It is used to compare frequencies between corpora of different sizes.
number of hits : corpus size in millions of tokens = frequency per million
Example: A token found 10 times in a corpus of 1 million tokens will have a frequency per million equal to 10. A token found 100 times in a corpus of 100 million tokens will have a frequency per million equal to 1. The second token is less frequent.