What is the operating temperature condition (Ta) indicated in a data sheet?

Common

The operating temperature condition in a datasheet is a ambient temperature when the IC itself is evaluated.
Basically, the IC is measured with restraining a self-heating. Therefore, a junction temperature of the IC is almost same as atmospheric temperature.
However, a measuring AC and Noise characteristic, etc take a long time. At that time, the junction temperature may not be always same as the ambient temperature as in these measurements.