[The first letter of electronic.]
The oldest form is e-mail, short for electronic mail, first recorded in 1982. From the early 1990s, others began to appear that signify types of information or transaction in digital electronic format, frequently transmitted over computer networks such as the Internet. It is a close relative of cyber-; however, each has its own constituency of compounds which do not much overlap.
Examples include e-cash and e-money (electronic analogues of money used for purchases online), e-banking (using the Internet to carry out banking transactions), e-auction, e-business, e-commerce, e-shopping (various Internet-mediated commercial activities). An e-book is one in electronic form which can be purchased and downloaded from the Net, an e-zine is a magazine distributed by e-mail, and an e-journal is an academic journal published in electronic form; e-text is a general term for a text made available in digital electronic form. Some are rhyming inventions: e-tailing (electronic retailing); e-lance, an electronic freelance, who works from home, relying on e-mail, fax, and telephone to stay in touch with clients.
Of these, only e-mail is at all commonly written without a hyphen, and the only one to have become a verb.