In plain terms
In the context of order flow, delta is the balance between two groups of executed volume. A common convention subtracts volume classified at the bid from volume classified at the ask.
How the term is used
A positive delta indicates more volume classified as buyer-initiated in the sample; a negative delta indicates the opposite. It does not mean that buyers exist without sellers: every trade has both counterparties. The calculation, a numerical example, and implementation variants are covered in Volume delta.
Limit
The side may come from the feed or be estimated by software, so two platforms can show different values. Delta measures executed trades, not quantity waiting in the order book, and it should not be confused with options delta.
Sources
- Sierra Chart, Numbers Bars — documents ask volume, bid volume, and their difference in Numbers Bars.
- TradingView, Volume Delta — shows a variant that estimates delta from lower-timeframe bars.