Armhole
The opening in the body of a garment where the sleeve is attached, and the curve on the pattern that forms it.
On the pattern, the armhole is a curve on both the front and the back pieces, and its length has to match the sleeve cap it joins (with the ease the sleeve needs). Its depth and width decide comfort and range of movement, and it is graded with its own rules from size to size.
Example
An armhole that is too shallow or tight restricts movement and pulls at the shoulder; one that is very deep lets the garment ride up whenever the arm is raised.