Gating is a method used in flow cytometry to isolate cells of interest by placing an electronic gate around cells that display specific fluorescent or light scattering characteristics. This electronic "gate" is usually created by the laboratory scientists analyzing the sample. Care must be taken to avoid placing the gate around too large of a cell population (which would lead to the inclusion of cells that don't belong to the population) or gating too small of a cell population (not including cells that should be included in analysis).
The white oval gate in the cytogram below is drawn around the lymphocyte population in the tested sample. This selection of a specific cell population is important because the extrinsic/fluorescent CD marker data (shown in the histogram on the right) directly reflects the population "gated" in the cytogram.