H

ETA

["This Greek letter comes right before theta and is often used to represent efficiency in science."]

The letter "H" is the eighth letter of the English alphabet, and "eta" is the seventh letter of the Greek alphabet. Since "H" is one position after "eta" in their respective alphabets (8 follows 7), the clue cleverly hints at the Greek letter as the answer. It’s a simple but clever numerical play on alphabet positions.