A drive-by download refers to any download to a computer or device that occurs without the owner’s consent. A drive-by download may occur simply from visiting a website or opening an email. Drive-by downloads can be spyware, adware, malware or even a legitimate program that the user simply doesn't want.
Techopedia explains Drive-By Download
Drive-by downloads from websites and email can usually be stopped by antivirus software, but other types of drive-by downloads are trickier to remove. For example, if an individual loans his or her computer or device to a friend who then downloads unwanted applications, this qualifies as a drive-by download. Sometimes a user will install a program that he or she wants, only to find that a host of unwanted programs have been installed along with it. Drive-by downloads are annoying, but they can quickly become infuriating it there is no obvious way to uninstall them.