Note 1: The wavelength may be expressed in radians or in artificial units of angular measure, such as degrees. Note 2 : In both coaxial cables and optical fibers, the velocity of propagation is approximately two-thirds that of free space[?]. Consequently, the wavelength will be approximately two-thirds that in free space, and the electrical length, approximately 1.5 times the physical length.
2. Of a transmission medium, its physical length multiplied by the ratio of (a) the propagation time of an electrical or electromagnetic signal through the medium to (b) the propagation time of an electromagnetic wave in free space over a distance equal to the physical length of the medium in question.
Note: The electrical length of a physical medium will always be greater than its physical length. For example, in coaxial cables, distributed resistances, capacitances and inductances impede the propagation of the signal. In an optical fiber, interaction of the lightwave with the materials of which the fiber is made, and fiber geometry, affect the velocity of propagation of the signal.
3. Of an antenna, the effective length of an element, usually expressed in wavelengths.
Note 1: The electrical length is in general different from the physical length.
Note 2: By the addition of an appropriate reactive element (capacitive or inductive), the electrical length may be made significantly shorter or longer than the physical length.
Source: from Federal Standard 1037C