An Access Control List (ACM) is a one-dimensional array that stores access rules for files. An ACM will be either (1) a list of all the users who can access a specific file, or (2) a list of all the files that can be accessed by a specific user (it depends on what the OS developers prefer it to be.)
"Figure 2-13: Access Control List" (page 81), Pfleeger, Charles P., et al. Security in Computing. Prentice Hall, 2015.