A disk that contains a kernel of some description.

The PC BIOS believes any disk is bootable if the last two bytes of the first sector of the disk are 0x55AA, it then jumps to the beginning of the sector and starts executing whatever code is there. Normally all disks are bootable, however they just contain the message

Non-system disk or disk error Replace and strike any key

and some code to reload the boot sector and try again after, of course you have pressed a key. This is why if you put an unformatted floppy in a machine that's set to boot off floppy before the HDD, it will normally skip booting the floppy and boot off the harddisk. However, now days it's getting pretty near impossible to find an unformatted floppy. :)