From The Collaborative International Dictionary of English v.0.48:
Arris \Ar"ris\, n. [OF. areste, F. ar[^e]te, fr. L. arista the
top or beard of an ear of grain, the bone of a fish.] (Arch.)
The sharp edge or salient angle formed by two surfaces
meeting each other, whether plane or curved; -- applied
particularly to the edges in moldings, and to the raised
edges which separate the flutings in a Doric column. --P.
Cyc.
[1913 Webster]
Arris fillet, a triangular piece of wood used to raise the
slates of a roof against a chimney or wall, to throw off
the rain. --Gwilt.
Arris gutter, a gutter of a V form fixed to the eaves of a
building. --Gwilt.
[1913 Webster]