"Stereo", as a word, refers to something that is solid or three dimensional. Its use however now includes any kind of playback system; for example, do you have a stereo? asked between new college roommates.

I believe consumers now assume that any playback system with more than one speaker or with headphones plays back at least two different channels - that is, a minimum of a left and right channel. I think this is a fair assumption as I don't believe we can find a mono portable player anywhere.

Somee use "stereo" to refer to any multi-channel playback, including home theater setups. I find this to be a bit confusing as I think of two channel as "stereo". However, I believe the use of the term is correct for any number of channels greater than two that creates the perception of a solid soundstage.

As usual, Elk has provided the accurate and complete response - including, this time, a reference to the Greek root of the term. As he said, iPods and the other portable players do provide a two channel (stereo) signal. Now if you put some great old mono recordings on your iPod (as some of us have) then the two channels for them will obviously be identical.

A somewhat off-topic comment:

Cheapskate's reference to mono recordings reminds me that I often find mono recordings to possess amazing "solidity" and presence. This is true whether the playback system is just one speaker or an in-phase pair (or headphones). There is a lot to be said for mono.

P.S. I love the idea of listening to mono recordings on an iPod; the earliest and the latest forms of music listening meet!

As an aging Brit returning to this world...I can recall articles in mags telling us all about Western Electric and the 'new' technology...lost me most of the time, however I always think of the word as being something recorded on two microphones at the same time and capable of being played back into both my ears at the same time...sometimes alternating between left and right and sometimes providing a huge panorama of sounds coming from somewhere in the middle...very confusing at the time after years of one single sound!

The replies so far are great and I guess these days the word itself is redundant as sounds can ceratinly go well beyond two inputs or outputs - the question these days must surely be how many sounds can two ears actually take in at any one time...from side to side 'stereo' where my head just flopped left and right, to the all around spinning where sounds start in speaker 'one' and go all the way around in sequence to 'seven' (or more!) with emphasis on whichever aspect at various points of the sound compass...bring back simple stereo???

(Well maybe not, but memories cannot be bad thing now and again...)