Refer to the figure on the bottom right plotting the heat capacity as a function of temperature. In this context, T is the temperature corresponding to point A on the curve.
Different operational definitions of the glass transition temperature T are in use, and several of them are endorsed as accepted scientific standards. Nevertheless, all definitions are arbitrary, and all yield different numeric results: at best, values of T for a given substance agree within a few kelvins. One definition refers to the viscosity, fixing T at a value of 10 poise (or 10 Pa·s). As evidenced experimentally, this value is close to the annealing point of many glasses.
In contrast to viscosity, the thermal expansion, heat capacity, shear modulus, and many other properties of inorganic glasses show a relatively sudden change at the glass transition temperature. Any such step or kink can be used to define T. To make this definition reproducible, the cooling or heating rate must be specified.
The most frequently used definition of T uses the energy release on heating in differential scanning calorimetry (DSC, see figure). Typically, the sample is first cooled with 10 K/min and then heated with that same speed.
Yet another definition of T uses the kink in dilatometry (a.k.a. thermal expansion): refer to the figure on the top right. Here, heating rates of  are common. The linear sections below and above T are colored green. T is the temperature at the intersection of the red regression lines.
Summarized below are T values characteristic of certain classes of materials.