According to etymologists, the term pea was taken from the Latin pisum, which is the latinisation of the Greek πίσον (pison), neut. of πίσος (pisos), pea. It was adopted into English as the noun pease (plural peasen), as in pease pudding. However, by analogy with other plurals ending in -s, speakers began construing pease as a plural and constructing the singular form by dropping the s, giving the term pea. This process is known as back-formation.

The name marrowfat pea for mature dried peas is recorded by the Oxford English Dictionary as early as 1733. The fact that an export cultivar popular in Japan is called Maro has led some people to assume mistakenly that the English name marrowfat is derived from Japanese.

Pea

• The pea is most commonly the small spherical seed or the seed-pod of the pod fruit Pisum sativum. Each pod contains several peas. Peapods are botanically a fruit, since they contain seeds developed from the ovary of a (pea) flower. However, peas are considered to be a vegetable in cooking. The name is also used to describe other edible seeds from the Fabaceae such as the pigeon pea (Cajanus cajan), the cowpea (Vigna unguiculata), and the seeds from several species of Lathyrus.

pea (n.)

• early or mid-17c., false singular from Middle English pease (plural pesen), which was both single and collective (as with wheat, corn) but the s sound was mistaken for the plural inflection. From Old English pise (West Saxon), piose (Mercian) pea, from Late Latin pisa, variant of Latin pisum pea, from Greek pison the pea, perhaps of Thracian or Phrygian origin [Klein].

Notice that the scientific name {genus) goes back to the original Latin.

Pea

Origin and meaning of pea by Online Etymology Dictionary

Pisum sativum (Common Pea)

Pea - Etymology