Three-ball
= Three-ball
{title2=$B^3$}
= 3-ball
{synonym}
A three-ball is a <three-manifold> <homeomorphic> to the closed unit <ball> in $\mathbb R^3$. Its <boundary> is a <two-sphere>. Attaching a three-ball to a spherical boundary component is called capping that boundary; marked endpoints may be joined by a straight arc inside the cap.