A review of the language and parameters that define subvisual cirrus is provided, including a literature review and classic examples of observations. Complementary measurement techniques are discussed, including lidar, solar aureole measurements, photographic techniques, and the use of radiometric observations. We address the microphysical and optical properties of these high, thin clouds, along with suggested dynamic sources. Five classes of subvisual cirrus clouds are suggested. Complementary subvisual cirrus observations recorded during the 1986 and 1991 FIRE (First ISSCP Regional Experiment) intensive field operations and other field programs support these classes. Detection of subvisual cirrus via satellite imagery is vital and a limiting case for discrimination or threshold methods. The lidar, satellite, rawinsonde, and meteorological conditions are used to derive a likely picture of the atmospheric conditions required to produce the subvisual cirrus class observed.