Wiktionary
n. 1 A branch of information security applied to both theoretical and actual computer systems. 2 A branch of computer science that addresses enforcement of 'secure' behavior on the operation of computers.
Wikipedia
Computer security, also known as cybersecurity or IT security, is the protection of information systems from theft or damage to the hardware, the software, and to the information on them, as well as from disruption or misdirection of the services they provide.
It includes controlling physical access to the hardware, as well as protecting against harm that may come via network access, data and code injection, and due to malpractice by operators, whether intentional, accidental, or due to them being tricked into deviating from secure procedures.
The field is of growing importance due to the increasing reliance on computer systems and the Internet in most societies, wireless networks such as Bluetooth and Wi-Fi - and the growth of "smart" devices, including smartphones, televisions and tiny devices as part of the Internet of Things.