What is Theta constant?

In mathematics, a theta constant or Thetanullwert' is the restriction θm = θm of a theta function θm with rational characteristic m to z = 0. The variable τ may be a complex number in the upper half-plane in which case the theta constants are modular forms, or more generally may be an element of a Siegel upper half plane in which case the theta constants are Siegel modular forms. The theta function of a lattice is essentially a special case of a theta constant.