The sending of unwanted emails (advertising\commercial ones mostly) to people who have not opted in (given their consent), and thus cluttering up their mailbox.
Sometimes the words Junk-Mail might be used to describe the phenomenon.
Summary of Socio-Legal aspects
Criminal behaviour: art. 18 (on the relationships with the Public Administration), art. 19 (data treatment concerning non-judicial and non-sensitive information), art. 23 (rules concerning consent), art. 123 (principles concerning call traffic), art. 126 (user location), art. 130 (unwanted correspondence) and art. 167 (unlawful handling of data) of the Testo unico sulla privacy, Law 196 del 2003.
Concerning the issue of opt in (and valid consent): D.lg. n. 171 del 1998, European Union Directive n. 2002/58/CE, published in the G.U.C.E. n. L 201, 31 July 2002.
It is necessary to specify that rarely children perpetrate this behaviour.