From The Collaborative International Dictionary of English v.0.48 :
Usable \Us"a*ble\, a.
Capable of being used.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
usable
adj 1: capable of being put to use; "usable byproducts" [syn:
useable, usable]
2: fit or ready for use or service; "the toaster was still
functional even after being dropped"; "the lawnmower is a bit
rusty but still usable"; "an operational aircraft"; "the
dishwasher is now in working order" [syn: functional,
usable, useable, operable, operational]
3: convenient for use or disposal; "the house is available after
July 1"; "2000 square feet of usable office space" [syn:
available, usable, useable]