Spline Joint

Joinery is a part of woodworking that involves joining together pieces of wood, to produce more complex items. Some wood joints employ fasteners, bindings, or adhesives, while others use only wood elements. The characteristics of wooden joints - strength, flexibility, toughness,appearance, etc. - derive from the properties of the joining materials and from how they are used in the joints. Therefore, different joinery techniques are used to meet differing requirements. For example, the joinery used to build a house is different from that used to make puzzle toys, although some concepts overlap.

A spline joint can be viewed as an extended biscuit. Two boards have extended, matching grooves lined up and facing each other. The void between is filled with a thin piece of wood, forming a spline joint. This is very similar to tongue and groove. The difference is that the spline essentially forms a tongue, a 'loose tongue', for both grooves. It is often joined with glue.

source; Wikipedia.org