The number of channels, or practically, the number of speakers supported in the setup.
7.1 means 1 centre speaker, front left and right speakers, left and right speakers, rear left and right speakers and 1 subwoofer.
7.2 just has another subwoofer.

Actually the .2 or .1 is for the bass sub woofer. Two or one powered speakers that cover 20 Hz to about 150 Hz, bass pedals on a pipe organ. Bass sounds do not have left or right but the extra speaker for .2 is for larger rooms, like 16 by 16 feet.

The 5. main speakers a left front, right front, center front, left side and right side ( side speakers are at ear level and position ).

The 7. has two more speakers at the rear. Placed in left and right position behind you.

There 9.2 and more coming . . .

OIC, thanks @ragingloli and @Tropical_Willie. Hmmmm… 9.2…. Would that be ceiling speakers?
OK, that triggers another thought. Is a 7.1 system really that much better than a 5.1? I mean, in order to hear the advantage to having 7 channels, your source must have 7 distinct channels, no? I think that ordinary CDs, DVDs, and Internet streaming don’t have 7 channels, therefore 7.1 is a waste of money, right?

The 5.1, 7.1 et al are for Blu-ray and beyond. Regular DVD’s don’t have the extra channels in the disc. You are correct DVD’s and regular TV are missing the extra channels. If you have HD though from a cable/satellite or over the air TV HD you will have the channels in there somewhere.

What happens if I, say, stream Netflix, or try Comcast Streampix, and they don’t carry the extra channels? Would the extra speakers just be silent?

The HD 5.1 receiver will fill-in the sound found for the right or left channels. It will not be any different than a two channels / speakers TV. But the BASS > > > > W O W !

@Tropical_Willie Sounds great (pun intended). My question is, suppose my sound source is only 2.1. Do the extra speakers remain silent if play this source, or do they all try to “fill-in”?

I used to hear the term “surround sound” that I don’t seem to hear much of anymore. Is surround sound 5.1 or 7.1?

Surround sound, yup from the 70’s. Also known a “Quad” only 4 channels used for music / audio. Modern 5.1 or 7.1 for movies and different with center channel ( dialogue ) included. The sound will come out of ALL speakers in 5.1 or 7.1 but will be “stereo” not surround or 5.1 ).

All GAs, @Tropical_Willie. Thank you for your input. I’m now better equipped to begin searching the Internet. I’ll know what to begin looking for.

I used to be the head projectionist at a movie theater and later went around Oregon fixing projectors in movie theaters. I quit in 1999 so my info is dated and could have changed.

Dolby Digital and DTS are both 5.1 channels. There is a center, left, right, left surround ,right surround and sub. The sub is .1 since it only covers a fraction of the frequency range.

Then there is SDDS (Sony Dynamic Digital Sound). It is 7.1 since it includes extra speakers between the thee stage speakers. Left-center and right-center. About 5% of the film prints came with a SSDS soundtrack. The equipment was insanely expensive (25K per theater for the equipment that sent the sound into the 15K cinema processor) so SDDS was only a last resort. And they put the audio on the very edges of the film which was prone to damage. Here is a pic of the soundtracks on a 35mm print. From left to right: SDDS, SR-D, Analog, and the morse code looking stuff is DTS. With DTS that isn’t the audio but a timecode. The Audio was on CDs.

I guess my rambling is saying that most film will probably not ship with a 7.1 soundtrack.

And fun fact, Dolby Digital read around 100K ones and zeros per second for audio. Shining a light through a piece of film traveling at 1.5 feet per second and recording that data was a fucking nightmare.