Q Is Windows Defender an Antivirus, or an Anti-Malware?
A virus is a subset of malware. Malware is defined a hostile or intrusive software, and includes: Viruses, Trojan horses, Rootkits, Backdoors, Worms, perhaps more.
The terms antivirus and anti-malware are often used interchangeably. If we are using the strict definition of the terms, antivirus is designed to locate and remove viruses, while anti-malware is designed to locate and remove viruses, trojans, rootkits, backdoors, worms, etc.
If we use strict definitions, Windows Defender is an antivirus. Products such as Bitdefender are anti-malware.