桁架（truss）为工程名词，是指「只由二力组件组成，组装后如同单一物体」的结构。二力组件（two-force member）是指只在二个端点上有受力的结构组件。在此严格的定义下允许以组件以稳定的组态组合出任意的形状，不过桁架一般会包括由直杆组件组合而成的结构，其中有五个或更多三角形单位，各组件的末端以结构节点相连接，称为顶点。

在此定义下，外力及因外力而产生的反作用力一般会视为只作用在端点上，而且只会让组件产生张力及压缩力，若都是直杆组件，不考虑各组件所受到的转矩，因为桁架中所有组件的结构节点都是旋转接点。一组件受到的转矩无法传递到其他组件。

In engineering, a truss is a structure that "consists of two-force members only, where the members are organized so that the assemblage as a whole behaves as a single object". A "two-force member" is a structural component where force is applied to only two points. Although this rigorous definition allows the members to have any shape connected in any stable configuration, trusses typically comprise five or more triangular units constructed with straight members whose ends are connected at joints referred to as nodes. In this typical context, external forces and reactions to those forces are considered to act only at the nodes and result in forces in the members that are either tensile or compressive. For straight members, moments (torques) are explicitly excluded because, and only because, all the joints in a truss are treated as revolutes, as is necessary for the links to be two-force members.