A line segment joining the midpoints of two sides of a triangle.
A triangle has 3 possible midsegments.

Try this Drag the orange dots on each vertex to reshape the triangle.
Notice the behavior of the midsegment line.

Properties

The midsegment is always parallel to the third side of the triangle.
In the figure above, drag any point around and convince yourself that this is always true.

The midsegment is always half the length of the third side. In the figure above, drag point A around.
Notice the midsegment length never changes because the side BC never changes.

A triangle has three possible midsegments, depending on which pair of sides is initially joined.