Market breadth is a measure of the number of stocks moving with the market averages. A move involving the majority is typically more robust than one in which a few heavily weighted stocks lift the averages. The former is reflective of wide spread institutional activity in the market.
Various measures of breadth may be used:
- Percentage of stocks above 50 day moving average
- Bullish percent index (percentage of stocks on PnF buy signals)
- Percentage of stocks up 25% in one month
Stockbee has a very detailed primer on market breadth here: