An endothermic reaction is a chemical reaction that absorbs energy from its surroundings.

Video segment. Assistance may be required. Watch the following video of an endothermic reaction between barium hydroxide octahydrate crystals with dry ammonium chloride.

Source: Endothermic reaction, trackerchem, YouTube

The heat absorbed is called enthalpy and is represented by ΔH.
In an endothermic reaction, energy can be represented as a reactant.

X + Y + 150kJ → Z

It can also be represented as having positive enthalpy.

X + Y → Z
ΔH = 150kJ

Breaking a chemical bond requires energy; therefore, it is an endothermic reaction.