1. the functional tissue of an organ as distinguished from the connective and supporting tissue: "the liver parenchyma"
▪ the cellular tissue, typically soft and succulent, found chiefly in the softer parts of leaves, pulp of fruits, bark and pith of stems, etc.: "xylem parenchyma cells"
▪ cellular tissue lying between the body wall and the organs of invertebrate animals lacking a coelom, such as flatworms.
Word Originlate 16th century: via Latin from Greek parenkhuma ‘something poured in besides’, from para- ‘beside’ + enkhuma ‘infusion’.
Parenchyma is the bulk of functional substance in an animal organ or structure such as a tumour. In zoology, it is the tissue that fills the interior of flatworms. In botany, it is some layers in the cross-section of the leaf. Wikipedia