1. Freedom from risk or danger; safety.
2. Freedom from doubt, anxiety, or fear; confidence.
3. Something that gives or assures safety, as:

1. A group or department of private guards: Call building security if a visitor acts suspicious.
2. Measures adopted by a government to prevent espionage, sabotage, or attack.
3. Measures adopted, as by a business or homeowner, to prevent a crime such as burglary or assault: Security was lax at the firm's smaller plant.

Why do we need security?

Protect vital information while still allowing access to those who need it
Trade secrets, medical records, etc.
Provide authentication and access control for resources
Ex: AFS
Guarantee availability of resources
Ex: 5 9 s (99.999% reliability)