In thermodynamics, you typically divide up the universe into two parts: the system and the surroundings. The system is everything you care about- the chemicals that are reacting, for example. The surroundings are everything else. During many processes, heat flows from the system to the surroundings, in others heat flows from the surroundings to the system.

We are free to choose what sign we want to assign to q, the quantity of heat flow. In thermodynamics, we choose a positive q to mean that heat flows from the surroundings to the system, and a negative q to mean that the heat flows from the system to the surroundings. The former is called an endothermic reaction: the latter is an exothermic reaction.

Summary

Sign of q	Direction of heat flow	Type of reaction
+	From surroundings to system	Endothermic
-	From system to surroundings	Exothermic

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