Water potential: The physical property predicting the direction in which water will flow governed by solute concentration and applied pressure.
Water vascular system: A network of hydraulic canals unique to echinoderms that branches into extension called tube feet, which function in locomotion, feeding, and gas exchange.
Wave of advance: A favorable allele advances behind a moving cline, known as a wave of advance.
Wavelength: The distance between crests of waves, such as those of the electromagnetic spectrum.
Weight: A system of units for expressing heaviness or mass, the amount or quantity of heaviness or mass.
Wetland: An ecosystem intermediate between an aquatic one and a terrestrial one. Wetland soil is saturated with water permanently or periodically.
White blood cell: A blood cell that function in defending the body against infections and cancer cells; also called leukocyte.
White matter: Tracks of axons within the central nervous system.
Wild type: The commonest allele at a locus or the most common phenotype.
Wilting: The drooping of leaves and stems as result of plant cells becoming flaccid.
Wobble pairing: The ability of tRNA to hybridize with codons even when only the first two nucleotides follow the standard G‑C and A-U base-pairing rules.