What is the longest Greek word?

The longest word in the Greek language is "antidisestablishmentarianism". It has 28 letters and means "opposition to the separation of church and state."

However, it's important to note that this word is not actually a native Greek word. It's a loanword from English, adopted into Greek and adapted to fit Greek spelling conventions.

For true Greek words, the longest ones are typically compound words, like "περιπλανώμενος" (periplanómenos), which means "wandering," and "αὐτοκινήτων" (autokinítōn), which means "of automobiles," each having 14 letters.