What is a stock scanner?
A stock scanner is a software tool that watches a universe of stocks in real time and surfaces the ones meeting specific conditions — price moves, volume spikes, indicator crosses, breakouts. It differs from a screener: a screener takes a static snapshot; a scanner watches and alerts continuously.
More detail
Stock scanners typically run continuously during market hours, refreshing as often as every second on premium services.
Most retail scanners offer pre-built scans (gainers, breakouts, volume movers) plus custom filters.
Real-time scanners require real-time data feeds — many 'free' scanners use 15-minute delayed data, which limits their usefulness for active trading.