What is ClamAV?

Clam AntiVirus (ClamAV) is a free and open-source, cross-platform antivirus software tool-kit able to detect many types of malicious software, including viruses. One of its main uses is on mail servers as a server-side email virus scanner. The application was developed for Unix and has third party versions available for AIX, BSD, HP-UX, Linux, OS X, OpenVMS, OSF (Tru64) and Solaris.

ClamAV includes a number of utilities: a command-line scanner, automatic database updater and a scalable multi-threaded daemon, running on an anti-virus engine.

Source: Installing And Scanning By ClamAV On Ubuntu 14.04 LTS | Unixmen