Greek deka, ten.
A decade is a period of ten years; a decathlon (Greek athlon, contest) is an athletic contest of ten events; a decagon is a plane figure with ten straight sides and angles. Decane is a colourless liquid hydrocarbon of the alkane series, whose molecule contains ten carbon atoms; a decapod (Greek pous, pod‑, foot) is a crustacean with five pairs of walking legs, such as shrimps and crabs. The form appears in units of measurement (decalitre, decametre) and is one of the standard SI (Système International) multiples (see the entry Number words); the standard specifies that it should be spelled deca‑. The spelling with k is used in the US. See also the next entry.