Shellcode is a small piece of code used by attackers to gain control over a computer system. Typically written in machine code, shellcode is often injected into a vulnerable program to execute commands, download additional malware, or open a backdoor for further exploitation. The name comes from its original purpose: to open a command shell on the target system. Today, shellcode can perform a variety of malicious actions, making it a crucial tool in many cyberattacks and exploits.