What does Sena mean in greek?

There isn't a word "Sena" with a standard meaning in classical or modern Greek. It's possible:

* It's a misspelling: Perhaps you're thinking of a similar-sounding word, or a word from another language.

* It's a proper noun: It could be a name (either given or place name) with no specific Greek etymology.

* It's a modern coinage or a regional dialect word: Greek, like any living language, evolves, and new words appear.

To get a more precise answer, you'd need to provide more context about where you encountered the word "Sena."