Step 1: Understand the function of diethyl ether.
Diethyl ether, commonly known simply as ether, was historically used as an anaesthetic for surgeries due to its ability to cause unconsciousness and loss of sensation.
Step 2: Evaluate the options.
- Option (A) Pain killer: Incorrect, while diethyl ether can relieve pain indirectly by inducing unconsciousness, it is not primarily classified as a pain killer.
- Option (B) Hypnotic: Incorrect, although it has hypnotic (sleep-inducing) properties, the term "hypnotic" is more commonly associated with substances specifically designed to induce sleep.
- Option (C) Antiseptic: Incorrect, diethyl ether has antiseptic properties, but it is not commonly used for this purpose.
- Option (D) Anaesthetic: Correct, diethyl ether is most widely known and used as an anaesthetic, particularly in the past for surgeries.
Final Answer:
\[
\boxed{\text{Anaesthetic}}
\]