The term "Dropper" in cybersecurity refers to a small piece of software used in malware deployment that is designed to install or "drop" malware (like viruses, ransomware, spyware) onto the target system.
The Dropper itself is not typically malicious in behavior; however, it is used as a vehicle to install malware that will perform malicious activities without detection.
During the infection process, the Dropper is usually the first executable that runs on a system. It then unpacks or downloads additional malicious components onto the system.
Reference
Common Malware Enumeration (CME): http://cme.mitre.org
Microsoft Malware Protection Center: https://www.microsoft.com/en-us/wdsi