en-2 Also em-.
Words in this prefix were mostly created in Greek. Em- is a variant used before b, m, p, and ph.
Examples created in classical Greek include: energy (Greek energeia, from ergon, work); enthusiasm (Greek enthousiasmos, from enthous, possessed by a god, based on theos, god); empathy (Greek empatheia, from pathos, feeling); enema (Greek enienai, send or put in, from hienai, send); endemic (Greek endēmios, native, based on dēmos, people).
Examples formed on Greek roots include the thermodynamic quantity entropy (Greek tropē, transformation); and engram (Greek gramma, letter of the alphabet), a hypothetical permanent change in the brain accounting for the existence of memory; a memory trace.