It stands for short-term trading index; a short-term technical indicator (also known as Arms index or Arms short-term trading index) that measures the strength of the volume of advancing stocks relative to the strength of declining stocks. It is calculated as:

An index reading of more than one means more activity in declining stocks, whilst a reading of less than one indicates more activity in advancing stocks.

This index is a contrary indicator that trends in the opposite direction of the market. However, a 10-day moving average of this index is a workable way to better grasp its contrary nature.