Define an active volcano.

Step 1: Define an active volcano.
An active volcano is a volcano that has erupted in recent history or shows signs of potential eruption in the near future, such as seismic activity or gas emissions.

An active volcano is one that has erupted within historical times or is currently showing signs of unrest, such as seismic activity, ground deformation, or gas emissions.

The final answer is $\boxed{\text{An active volcano is a volcano that has erupted in recent history or shows signs of potential eruption in the near future, such as seismic activity or gas emissions.}}$