* ειδικός (eidikós): This is probably the closest single word and means "specialist" or "expert" in a specific field.
* τέχνης (téchnēs): This means "art" or "craft," and could be used to describe someone skilled in a particular area, implying expertise. You might see phrases like "τέχνης μάστορας" (téchnēs mástoras) meaning "master craftsman," conveying expertise.
* δαίμων (daimōn): While literally meaning "spirit" or "divinity," this could be used figuratively to describe someone exceptionally skilled or gifted in a particular area, almost implying a supernatural level of expertise. This is less common and more poetic.
The best choice depends heavily on the nuance you want to convey. For most situations, ειδικός (eidikós) is the most accurate and commonly used translation.