A segment is one of the individual, membrane-wrapped sections of juicy pulp and seeds that a citrus fruit naturally divides into, such as in oranges, grapefruits, lemons, and limes. The noun refers to the piece of fruit itself; "segmenting" is the knife technique used to free those segments from their skin, pith, and membranes.
Segments are added whole or halved to salads, desserts, and cocktails for a burst of citrus flavor without extra prep. Grapefruit segments run larger and more bitter-sweet than the sweeter segments of oranges or mandarins; blood oranges and Cara Cara oranges add color.