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a tangle is generally one of two related concepts:

In John Conway's definition, an n-tangle is a proper embedding of
the disjoint union of n arcs into a 3-ball;
the embedding must send the endpoints of the arcs to 2n
marked points on the ball's boundary.

In link theory, a tangle is an embedding of n arcs and m circles
into R^2 X [0,1] – the difference from the previous definition
is that it includes circles as well as arcs,
and partitions the boundary into two (isomorphic) pieces,
which is algebraically more convenient –
it allows one to add tangles by stacking them, for instance.