Jizz is a term originally used by birdwatchers to describe the overall impression or appearance of a bird garnered from such features as shape, posture, flying style or other habitual movements, size and colouration combined with voice, habitat and location. The concept is so useful however, that it since has been adopted increasingly widely by field biologists in referring to the impression of the general characteristics of other animals. It similarly appears in such fields of observational biology as microscopy. Ecologists and botanists may speak of "habitat jizz" or the jizz of a plant.
Sean Dooley described jizz as "the indefinable quality of a particular species, the 'vibe' it gives off" and notes that although it is "dismissed by many as some kind of birding alchemy, there is some physical basis to the idea of jizz."
Experienced birders can often make reliable identifications in the field at a glance by utilising jizz. Often jizz is useful for identifying to the family or genus level, rather than the species level. For example, "It definitely had the jizz of a thrush, but I couldn't see what kind."
The origin of the word is a mystery. There is a theory that it comes from the World War II air force acronym GISS for "General Impression of Size and Shape (of an aircraft)", but the birding term was first recorded earlier than that in 1922. More likely, jizz is a corruption of gestalt, a German word that roughly means form or shape. Other possibilities include the word gist, or a contraction of just is.