Square cut two pipe ends. Now place them end to end as a joint. If subsequently welded, this would be a girth weld joint
The cut plane (section) of the pipe end describes a circle. Girth describes this [general] circular shape/geometry. Girth: around the perimeter.
Oblique angle joints between pipe are also girth welds (section shape here is oval, hence generalized use of term "girth"). Joining of large formed cylinders (## meters diameter), as for pressure vessels are also girth welds. The practical aspects of pipe or vessel girth welding may be different and are context specific.
Hope clear.