In differential geometry, the second fundamental form (or shape tensor) is a quadratic form on the tangent plane of a smooth surface in the three-dimensional Euclidean space, usually denoted by I I {\displaystyle \mathrm {I\! I} } (read "two"). Together with the first fundamental form, it serves to define extrinsic invariants of the surface, its principal curvatures.
In differential geometry, the second fundamental form (or shape tensor) is a quadratic form on the tangent plane of a smooth surface in the three-dimensional Euclidean space, usually denoted by I I {\displaystyle \mathrm {I\! I} } (read "two"). Together with the first fundamental form, it serves to define extrinsic invariants of the surface, its principal curvatures.
Source: Wikipedia "Second fundamental form" · CC BY-SA 4.0
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