English speakers often use THE plus a singular noun when they talk about or make generalizations about certain topics, including:
1. musical instruments (the piano, the guitar, the flute)
2. plants (the coconut palm, the saguaro, the baobab)
3. animals (the leopard, the elephant, the lowland gorilla)
4. inventions (the steam engine, the plane, the light bulb)
5. currencies (the dollar, the euro, the yen)
6. body parts (the head, the eye, the ear)

Examples:
1. I play the piano.
2. The sequoia tree is native to California.
3. The dolphin is a very intelligent animal.
4. The Wright brothers invented the airplane.
5. Right now, the euro is stronger than the dollar.
6. Cheryl got poked in the eye.