Activation energy or Energy or activation.

Explanation

**Step-by-Step Text Solution:**

1. **Definition of Activation Energy:**
Activation energy (Ea) is defined as the minimum amount of energy required for a chemical reaction to occur. It is the energy barrier that must be overcome for reactants to be transformed into products.

**Hint:** Think of activation energy as the "hurdle" that reactants must jump over to start a reaction.

2. **Energy of Reactants:**
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