1. [common among security specialists] A cracker, someone bent on breaking into the system you are protecting. Oppose the less comon white hat for an ally or friendly security specialist; the term gray hat is in occasional use for people with cracker skills operating within the law, e.g. in doing security evaluations. All three terms derive from the dress code of formulaic Westerns, in which bad guys wore black hats and good guys white ones.
2. [spamfighters] ‘Black hat’, ‘white hat’, and ‘gray hat’ are also used to denote the spam-friendliness of ISPs: a black hat ISP harbors spammers and doesn't terminate them; a white hat ISP terminates upon the first LART; and gray hat ISPs terminate only reluctantly and/or slowly. This has led to the concept of a hat check: someone considering a potential business relationship with an ISP or other provider will post a query to a NANA group, asking about the provider's hat color. The term albedo has also been used to describe a provider's spam-friendliness.