In electronics terminology, the word "hot" is often used to mean "active" or "powered on." Therefore, a hot swappable device is a peripheral or component that can be removed or added while a computer is running. Replacing a device while a computer is powered on is called "hot swapping."
Most early I/O devices were not hot swappable. This is because early ports, such as parallel ports and SCSI ports did not support connecting or removing devices while the computer was powered on. In fact, removing devices from these older ports while the computer was running could cause damage to them or the computer. Since it was hassle for users to turn off their computers each time they needed to connect or reconnect a device, newer I/O ports were designed to be hot swappable. Modern ports that support hot swapping include USB, FireWire, and Thunderbolt.
Most modern PCs include only hot swappable ports. Therefore, the term is now used most often to describe internal components that can be replaced while the computer is running. A common example is a server hard drive. Since servers need to run constantly to avoid downtime, they typically support hot swappable hard drives. Rack-based servers often provide easy access to one or more hard drives on the front face of the rack unit. This allows server administrators to quickly add more storage or replace old hard drives without powering down the server.
NOTE: Except for rare exceptions, RAM is generally not hot swappable.