Avast virus definitions for Windows are essentially

databases of known malware signatures and characteristics

. They are regularly updated to include new threats discovered by Avast[SQ]s researchers. These definitions are crucial because they allow Avast antivirus software to :

*

Identify malware:

By comparing files and processes on your computer to the signatures in the definitions, Avast can detect and flag known viruses, Trojans, worms, ransomware, and other malicious software.
*

Block malware:

Avast can prevent malicious code from running or installing, protecting your system from infection.
*

Remove malware:

If malware is detected, Avast can remove it from your system, often with the help of the virus definitions that provided the initial identification.

Think of it like a constantly updated "wanted poster" database for cybercriminals. The more up-to-date the database (the virus definitions), the better Avast can protect your Windows computer. The definitions are downloaded automatically by Avast in the background unless you manually disable automatic updates.