Stationary

Fixed, unmoving. Generally related to relationships between data series. Stationary relationships hold true for long periods of time, perhaps permanently.

Steady State

A process running at one set of conditions without significantly varying.  All data samples taken at a given steady state condition represent repeated samples of one data point and their variance express test and measurement error.

Training Data

Also known as "Modeling Data", this data used by the modeling process to calculate and set model parameters and coefficients. This data is usually highly fit by the models.