Final answer:
Two systems at thermal equilibrium share the same temperature and experience no net heat transfer between them; this follows from the zeroth law of thermodynamics.
Step-by-step explanation:
When we discuss two systems at thermal equilibrium, we are referring to a condition where both systems share the same temperature and no net heat transfer occurs between them. This concept stems from the zeroth law of thermodynamics, which is a fundamental principle in physics. For example, if system A is in thermal equilibrium with system B, and B is in thermal equilibrium with a third system C, then by the zeroth law, A is also in thermal equilibrium with C. This principle helps us understand how energy is exchanged within systems until temperature uniformity is established, signifying thermal equilibrium.