Originally, a square wooden frame thrown on to the ground by, for example, botanists who wished to count plant species at random locations. Now used more generally to refer to a sampled area of space.
Biology
An ecological sampling unit consisting of a small square area of ground within which all species of interest are noted or measurements taken. Quadrats may be spaced over a larger area to form an overall view when a total survey would be impracticable, or they may be used to sample along a transect.