1. A band in the electromagnetic spectrum that offers maximum transmission and minimal attenuation through a particular medium with the use of a specific sensor. See atmospheric window. 2. Open water in a predominantly frozen river. It is caused by warmer water from a spring or tributary, or by turbulence over a shoal. See polyn'ya. 3. (Obsolete. ) A term for the chaff used for military countermeasure purposes during World War II.