It is lossless.

I'm not sure that's the right word for uncompressed digital audio, because it's lossless compared to what? Presumably an analog recording or the original input signal? Because Shannon-Nyquist, with CD audio you can't get anything higher than what? 16kHz out of it, but within that limitation you can reproduce any arbitrary waveform within a speaker's ability to produce given the laws of physics regarding inductance and inertia.

MP3 does use a lossy compression, but you can maintain listenable quality while cramming about 10 times as much audio into a given space. You can get just over an hour of Red Book audio on a CD, and about 11 hours of mp3s, give or take. You might get lower audio bandwidth or various kinds of artifacts but it'll still sound pretty good, it's way more practical to store and transmit over the internet. We didn't Napster no .wav files.

FLAC and similar formats use lossless compression, kind of like a .zip file. If you rip a CD to FLAC, and you were to then burn a CD from that FLAC, the data on the new CD would be identical to the old one. So you get as-perfect-as-we-can-do digital audio, but only 5 or 6 hours worth would fit on a CD. Someone somewhere on this earth has filled a compact disc with FLAC files, I'm sure.