A hacker is one who exploits systems or gains unauthorized access through clever tactics and detailed knowledge, that is, through the use of a hack. However, because most hacks do not exploit systems or gain unauthorized access, most people who have enough technical skill to produce clever hacks consider the use of the word hacker in this sense to be bigotry. Malicious hackers in this sense are often called black hat hackers, but it is more appropriate to call them crackers(from criminal hacker) as this is a term which distinguishes the exploitation of security weaknesses from hacking in general. The opposite term of black hat is white hat, are ethical hackers those who attempt to break into systems or networks in order to help the owners of the system by making them aware of security flaws, or to perform some other altruistic activity. Other hackers often viewed negatively include phreakers and software crackers.