In philately a killer is a particularly heavy type of

handstamp A postmark is a postal marking made on an envelope, parcel, postcard or the like, indicating the place, date and time that the item was delivered into the care of a postal service, or sometimes indicating where and when received or in transit. ...

, or portion of one, consisting of heavy bars, cork impressions or other crude devices used to cancel

postage stamps A postage stamp is a small piece of paper issued by a post office, postal administration, or other authorized vendors to customers who pay postage (the cost involved in moving, insuring, or registering mail), who then affix the stamp to the f ...

.Bennett, Russell and Watson, James; ''Philatelic Terms Illustrated'', Stanley Gibbons Publications, London (1978). Such handstamps may also be known as ''obliterators'' as the mark applied often obscures almost the whole of the stamp. Killers were often used in the early days of stamps as the postal authorities wished to ensure that stamps could not be re-used. In the United States this is also the name for a particular circular date stamp with four thick horizontal bars to the right. This handstamp effectively cancels the stamp while leaving the place and date information easily visible. The bars are known as ''killer bars''.Patrick, Douglas & Mary. ''The Hodder Stamp Dictionary'', Hodder & Stoughton, London, 1973, p.126. {{ISBN, 0-340-17183-9. There is no exact definition of what is, and is not, a killer cancel and the term is often used to apply to any heavy cancellation.

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Example of cork killer on cover. Image of a killer cancellation on an early US stamp.Linns refresher course ''U.S. handcancels prevented stamp reuse'' by Michael Baadke.
Postal markings Philatelic terminology