Plain English Meaning
How far apart the air temperature and the dewpoint are. When those two numbers get close together, the air is nearly full of moisture and fog or low cloud is likely to form.
Full Definition
The difference, in degrees, between the current air temperature and the current dewpoint temperature. As the spread narrows toward zero, the air is approaching saturation and visible moisture (fog, low cloud, or mist) becomes increasingly likely.
Context Anchor
Seen in weather reports, forecasts, and fog discussions when deciding how close the air is to forming fog or low clouds.
Derivation
‘Spread’ here simply means the gap between two values. It is borrowed from everyday usage — the spread between two numbers — and applied to the temperature and dewpoint readings.