What is meant by "two-way" audio?

[RrCcGg 0]

When a camera indicates this, is it describing stereo microphones or some bi-directional audio (like mic + speakers) feature or something else?

[zacharyx 0]

means it is capable of audio both in and out, either through external microphone/speakers or built in to the unit.

[lv426 0]

Yup, it means you can talk and listen. Several of my cameras, mostly Foscams, have 3.5mm audio jacks allowing the use external mics & speakers. Depending on how they are being viewed there can be significant delay (well 1 or 2 seconds), fine for answering a caller at your door for example...