The glass transition temperature (Tg) is a crucial concept in material science, defining the temperature range where an amorphous solid becomes more rubbery than glassy, and its mechanical properties change significantly. However, researchers and engineers often use different terms to describe Tg, depending on the field of study, the type of material, or the method of analysis. For instance, Tg is also called the vitrification temperature, the softening point, the relaxation temperature, or the transition region temperature. Moreover, related terms, such as the cold crystallization temperature, the fragility index, or the enthalpy relaxation, are often used to assess the physical, chemical, and mechanical behavior of glasses and polymers near their Tg.