The combining form atel/o means incomplete or imperfect. It originates from the Greek word atelēs, meaning "imperfect" or "incomplete". This prefix is often used in medical terms to describe conditions where something is not fully developed or expanded, such as atelectasis, which refers to the incomplete expansion or collapse of a lung. 

• Atel/o: Incomplete, imperfect 

Example: Atelectasis is the collapse of a lung or part of a lung, signifying an incomplete expansion.