Single span

A single span curve or surface is one that has the smallest number of control points for a given degree. This means; Degree +1 control points.

It’s ‘single span’ because there are no internal knots.

For a fuller description of the benefits of using single span, see Autocad's excellent documentation: Single span

Source: https://discourse.mcneel.com/t/basic-rebuild-problem/51612/29