Image quality (IQ) is what you make of it.
Seriously, it's the quality of an image delivered by the lens, and that's affected by several parameters. Many people equate IQ with sharpness, but it's important to recognize that sharpness is only one characteristic of IQ, and sharpness itself has multiple meanings. Other characteristics include contrast and color rendition. There are various distortions and aberrations that affect IQ - geometric distortion, spherical aberration, chromatic aberrations (lateral and axial), vignetting, flare, etc.
The recording medium also affects IQ - a lens does not function in isolation (although there are ways to test lenses in isolation, those are practically meaningless, since as a lens user, you need a camera to use the lens with). Higher MP sensors result in an increase in perceived sharpness, but also sometimes magnify the effects of aberrations.
Post-processing is another factor, involving yet more tradeoffs. Many of those aberrations listed above can be corrected by software (especially geometric distortion, lateral CA, and vignetting). However, some of those corrections result in a loss of sharpness in the corrected areas.
So...I guess what it boils down to is that a lens with great IQ takes pretty pictures.
I'd recommend not getting too hung up on lens sharpness - there are a lot of things you can do to increase the IQ of your images that don't depend directly on the lens. For example, if you're shooting static subjects, using a tripod helps enormously. Using a lens hood can increase contrast and color saturation in your images compared to the same lens without a hood. Etc.
Or, if you prefer, go buy yourself an ISO 12233-type chart that costs more than some L-series lenses, and go nuts... Heck, it works for me!
Those charts cost that much?