The Secure Shell (SSH) daemon. It is the server side process to which SSH clients can connect and invoke commands and services. Note that system administration questions are off-topic on this site.
SSHD is an acronym for Secure Shell Daemon, the OpenSSH SSH server. It is the server side process to which SSH clients can connect and invoke commands and services.
Use ssh instead if your question is not specifically about the daemon, but (also) about the client or the protocol.
Note that system administration is off-topic on Stack Overflow. If your question is not a programming question, ask on Unix & Linux (if running under Linux or other unix system), Super User (if you aren't a professional system administrator) or Server Fault (if you are a professional system administrator).
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The acronym is occasionally used to refer to Solid State Hard Drives, more commonly abbreviated ssd.
