The epiphysis is the rounded end of any long bone, adjacent to the growth plate, and which is the part involved in the actual joint.

The physis is the growth plate, and the epiphysis is the bone on the side of it which is part of the joint.
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CLINICAL PUBLICATIONS
Changes of the growth plate in children: 3-dimensional magnetic resonance imaging analysis. Yun HH, Kim H-J, Jeong M-S, Choi Y-S and Ji-Young Seo J-Y. Korean J Pediatr. 2018 Jul; 61(7): 226–230.