In computing, a blacklist is a basic access control mechanism that allows every access, except for the members of the black list (i.e. list of enied accesses). The opposite is a whitelist, which means allow nobody, except members of the white list. As a sort of middle ground, a greylist, contains entries that are temporarily blocked or temporarily allowed. Greylist items may be reviewed or further tested for inclusion in a blacklist or whitelist.