is a standalone program that drops a virus to a system. Usually a dropper for a file virus is a very small program (a few bytes) infected by a virus.
A dropper for a boot virus is usually a program that writes the image of a boot sector virus stored inside it to a hard or floppy drive.
Virus droppers are no longer widespread as malware with the same capabilities integrated are becoming more common. Malicious programs with dropper-like capabilities are now identified as Trojan-Droppers