Every superfield, i.e. a field that depends on all coordinates of the superspace, may be expanded with respect to the new fermionic coordinates. The generic kind of superfields, typically a vector superfield, indeed depend on all these coordinates. The last term in the corresponding expansion, namely D θ θ 1 θ θ 2 θ θ ¯ ¯ 1 θ θ ¯ ¯ 2 , is called the D-term.