That can be booted; from which one can boot the system.(adjective)
A disk or other medium from which the system can be booted.(noun)
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Use "bootable" in a sentence
"A so-called bootable clone made on a USB drive is not actually bootable - as you know."
"If your internal drive fails while your only backup is the so-called bootable clone on your USB drive, and you have no other disk with which to start your computer and access the Finder"
"Table 2 is applicable to an IBM PC-1, which is fitted with either a "bootable" harddisk or an IBM Expansion Chassis."