Market Breadth
The ratio of advancing stocks to declining stocks — a check on whether an index move is broad-based or narrow.
Market breadth (advances vs. declines) tells you how many individual stocks actually participated in a day's index move, not just whether the index itself went up or down. A Nifty 50 index can rise 1% on the back of just 3–4 heavyweight stocks while hundreds of other listed stocks fall — that's narrow, fragile breadth.
Broad breadth (a large majority of stocks advancing alongside the index) is generally read as healthier and more sustainable than a rally concentrated in a handful of names.
Example
1,124 advancing vs. 387 declining stocks is broad, healthy breadth. 600 advancing vs. 900 declining while the index is still up is a warning sign — the index gain is being carried by a few large stocks.