1Stereo — an audio format that uses two or more channels (typically left and right) to give sound a sense of spatial depth, as opposed to mono. The standard playback format for modern audio, used on CDs, streaming services, headphones, speakers, and so on.
この曲はステレオで聞くと音が広がって気持ちいい。
This song sounds great in stereo — the sound really opens up.
片方のイヤホンが壊れて、ステレオで聞けなくなった。
One of my earbuds broke, so I can't listen in stereo anymore.
ライブ音源はステレオよりもサラウンドで聞くほうが臨場感がある。
Live recordings feel more immersive in surround than in stereo.
古い番組はモノラル音声で、ステレオには対応していない。
The old show is in mono audio and doesn't support stereo.
2A home stereo system — the freestanding audio set (component stereo, ステレオセット) that was a fixture of Japanese homes from the 1970s through the '90s. In modern usage this meaning is dated: Bluetooth and smart speakers have largely replaced it, and standalone "ステレオ" in this sense survives mainly with older speakers or in second-hand / nostalgic contexts.
実家のリビングには昔ながらの大きなステレオが今も置いてある。
There's still an old-school big stereo set sitting in the living room at my parents' place.
古いステレオを処分して、Bluetoothスピーカーに買い替えた。
I got rid of the old stereo and replaced it with a Bluetooth speaker.
父は今でもレコードをステレオで聞くのが好きだ。
My dad still loves listening to records on his stereo.
3Used prenominally to mean "stereoscopic" — i.e., creating a sense of three-dimensional depth, especially visually. Common in compounds like ステレオ写真 (stereoscopic photo), ステレオ画像 (stereo image), ステレオ視 (stereoscopic vision). When ステレオ〜 prefixes audio terms, it's an extension of sense 1.
19世紀のステレオ写真を博物館で見て、当時の3D技術に驚いた。
I was amazed at the 19th-century stereoscopic photos I saw at the museum — that level of 3D tech back then.
子供のころ眼科でステレオ視の検査を受けた覚えがある。
I remember having a stereoscopic vision test at the eye doctor when I was a kid.