Only here in the New Testament. Originally, in a physical sense, the diaphragm. Denoting the reasoning power on the reflective side, and perhaps intentionally used instead of νοῦς (1Co 14:15), which emphasizes the distinction from ecstasy.
Children - be ye children (παιδία - νηπιάζετε)
The A.V. misses the distinction between children and babes, the stronger term for being unversed in malice. In understanding they are to be above mere children. In malice they are to be very babes. See on child, 1Co 13:11.