Beyond the five main Greeks sit second-order ones. Charm (also called "delta decay" or "delta bleed") is one: it measures how an option's delta changes purely from time passing, even if the stock doesn't move at all.
Why time alone matters
As expiry approaches, an option's delta drifts — an out-of-the-money option's delta bleeds toward zero, an in-the-money one toward one. So a hedger who is perfectly delta-neutral today can be off tomorrow morning without the stock having moved a cent. Charm is the number that predicts that drift.
The "surface"
Charm depends on strike and on time to expiry, so plotting it across all strikes and expiries gives a 3-D charm surface. Options market-makers watch it to know how much stock they'll need to buy or sell overnight or over a long weekend just to stay hedged — the hedge that time itself forces on them.
This is not a retail signal
You'll see "charm flows" invoked to explain market moves, especially around big monthly expiries. Some of that is real dealer hedging; a lot of it is narrative sold by flow-data services. Charm is a professional risk-management tool, not an edge you can trade at home — and we make no claim that watching it predicts prices. We include it because understanding options honestly means understanding what the pros actually track. We ourselves take options exposure only through real funds (see the methodology).
Educational material — not investment advice.