Definition
The curved upper surface of a liquid in a narrow tube or container, formed by the interaction of surface tension and the liquid's adhesion to the container walls. The curve may be concave (curving downward in the middle, as with water in glass) or convex (curving upward in the middle, as with mercury in glass). When reading a liquid level against a scale, the measurement is taken at a consistent point on the meniscus -- typically the bottom of the curve for water-based liquids and the top of the curve for mercury.
Plain English
The curved surface you see at the top of a liquid sitting in a narrow tube. To read the level accurately, you have to read it at the same point on the curve every time.
Context Anchor
Seen in maintenance, fuel testing, fluid measuring, and any procedure where a liquid level must be read accurately in a tube or marked container.
Derivation
From the Greek meniskos, meaning 'crescent' or 'little moon.' The curved liquid surface looks like a crescent moon when viewed from the side, which is why the name stuck.
Why Pilots Care
Reading a mercury barometer or a manometer incorrectly -- by sighting the wrong part of the curve -- gives a wrong pressure value. In aviation instruments calibrated against liquid columns, consistent meniscus reading is what makes the measurement trustworthy.
Analogy
A glass of water may look like the water climbs slightly up the side of the glass. That curved edge is the same kind of shape as a meniscus.
Grounding Statement
Look at the top of liquid in a narrow clear tube: if the surface bends up or down at the edge, that bend is the meniscus.
Intuition Check
Do not assume the liquid level is the highest or lowest point of the curve every time. Use the part of the meniscus specified by the procedure or measuring device.
Example Sentence 1
When reading the mercury barometer, sight the top of the meniscus to get an accurate pressure value.
Example Sentence 2
Surface tension caused the fuel in the sight gauge to form a meniscus that had to be accounted for during quantity checks.