evanescent wave
oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source
en-GB/ˌiːvəˈnɛsnt ˈweɪv/
en-US/ˌɛvəˈnɛsənt ˈweɪv/
oscillating electric and/or magnetic field that does not propagate as an electromagnetic wave but whose energy is spatially concentrated in the vicinity of the source