Why was Rome considered a republic?

It was called a republic because that is they type of government that ruled Rome during most of its existence.

The Roman Republic ... It all began when the Romans overthrew their Etruscan conquerors in 509 B. C. E. Centered north of Rome, the Etruscans had ruled over the Romans for hundreds of years. Once free, the Romans established a republic, a government in which citizens elected representatives to rule on their behalf.