letter bomb

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An explosive mailed in an envelope rigged to cause detonation upon being opened.

from the GNU version of the Collaborative International Dictionary of English.

• noun A bomb disguised as a letter and sent through the mail, usually rigged to explode and kill or harm the recipient when opened.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun An explosive device sent through the post, triggered to explode when the letter or packet is opened.
• noun computing A virus contained within an email or similar file.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb send an explosive to
• noun a thin explosive device inside an envelope or package and detonated when opened