In geometric optics, at a refractive boundary, the smallest angle of incidence at which total internal reflection occurs.
Note 1: The angle of incidence is measured with respect to the normal at the refractive boundary.
Note 2: The critical angle is given by where _c is the critical angle, n₁ is the refractive index of the less dense medium, and… (Read More →)

An operational area that requires specific environmental control because of the equipment or information contained therein.

1. In radio propagation by way of the ionosphere, the limiting frequency at or below which a wave component is reflected by, and above which it penetrates through, an ionospheric layer.
2. At vertical incidence, the limiting frequency at or below which incidence, the wave component is reflected by, and above which it penetrates through, an ionospheric layer.
Note: The… (Read More →)

1. Elements of a system that are so vital that disabling any of them would incapacitate the entire system.
2. [In security,] those physical and cyber-based systems essential to the minimum operations of the economy and government. [INFOSEC-99]

A mechanism within a Target of Evaluation whose failure would create a security weakness [ITSEC].

Synonym essential service.

That part of the total technical power load required for synchronous communications and automatic switching equipment.

The free-space wavelength that corresponds to the critical frequency.
Note: The critical wavelength is equal, in meters, to the speed of light (3 × 108 m/s) divided by the critical frequency in hertz.

Abbreviation for certificate revocation list. In cryptography, a list of revoked certificates.

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