Citrus aurantifolia Swingle - Key lime is a small, bushy tree, rarely taller than 12 feet (4.1 m). It has slender branches armed with short to medium-length thorns. Spineless selections are more compact and upright in growth, have darker green foliage, and are characterized by low fruit production.

Fruits are small, 1 ½ to 2 inches (38–51 mm) in diameter, round to oval with 10–12 segments around a small, solid axis. The peel is thin, smooth, leathery, greenish and yellow at maturity. The pulp is greenish-yellow, juicy, and highly acidic with a distinctive aroma.