1. The abuse of electronic messaging systems to indiscriminately send unsolicited bulk messages.
Source: Explore Terms: A Glossary of Common Cybersecurity Terminology, National Initiative for Cybersecurity Careers and Studies (NICCS), https://niccs.us-cert.gov/about-niccs/glossary
2. Electronic junk mail or the abuse of electronic messaging systems to indiscriminately send unsolicited messages.
Source: U.S. Cyberspace Solarium Commission, March 2020, https://subscriber.politicopro.com/f/?id=00000170-c638-d8f7-a7f1-f63b33510000
3. Unsolicited messages sent to a large number of recipients.
Source: An Investigation into Foreign Entities Who Are Targeting Servicemembers and Veterans Online, Vietnam Veterans of America, https://vva.org/wp-content/uploads/2019/09/VVA-Investigation.pdf
4. Spam refers to unsolicited and automatically sent mass advertising, into which category spam e-mails also fall. The person responsible for these messages is known as a spammer, whereas the actual sending itself is known as spamming.
Source: Information Assurance Situation in Switzerland and Internationally, Reporting and Analysis Centre for Information Assurance MELANI, https://www.newsd.admin.ch/newsd/message/attachments/11945.pdf