Step 1: Definition of Potential Evapotranspiration.
Potential evapotranspiration (PET) is the maximum evapotranspiration rate in a fully vegetated area with ample moisture. It represents the theoretical water loss under ideal conditions for evaporation and transpiration.
Step 2: Answer.
Therefore, PET is correctly defined as evapotranspiration with sufficient moisture in a fully vegetated area, making option (1) the correct answer.
Final Answer:
\[
\boxed{1. \, \text{Evapotranspiration when there is sufficient moisture available in a fully vegetated area}}
\]