The Greeks are a map of sensitivity

Ask what is changing

An option's value can change when the underlying price moves, when time passes, or when the market reprices volatility. The Greeks describe local sensitivities to these inputs, with other assumptions held fixed. They are useful approximations, not a promise of what the next trade will produce.

Delta describes price sensitivity. Gamma describes how that delta changes as the underlying moves. A position's behaviour therefore evolves; yesterday's delta is not a permanent label.

Time and volatility matter too

Theta describes sensitivity to the passage of time under the model's convention. Vega describes sensitivity to implied volatility. Be explicit about units: a model may quote a Greek per share or per contract, and volatility changes are often expressed in percentage points.

A position that appears directional can also carry substantial volatility and time exposure. Reading only one Greek can leave a large part of the position unexplained.

Use the map carefully

Write the direction and unit of each important sensitivity before using a number in a decision. Recheck after the market moves. For large moves, evaluate a full scenario rather than extrapolating a small-move approximation indefinitely.