homocercal fin (noun)

1. In Greek mythology, the homocercal fin was said to be a gift from Poseidon to the god of the sea creatures.

2. As the superhero leaped into the ocean, his powerful homocercal fin propelled him through the water with incredible speed.

3. The main character in the anime series had a unique ability which allowed her to summon a magical homocercal fin when she needed to traverse the underwater realm.

4. In the distant future, humans have genetically modified their bodies to have homocercal fins, enabling them to navigate the vast oceans of alien planets.