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This is a glossary of terms specific to differential geometry and differential topology. The following three glossaries are closely related:

Words in italics denote a self-reference to this glossary.

## B

• Bundle – see fiber bundle.
• basic element – A basic element ${\displaystyle x}$ with respect to an element ${\displaystyle y}$ is an element of a cochain complex ${\displaystyle (C^{*},d)}$ (e.g., complex of differential forms on a manifold) that is closed: ${\displaystyle dx=0}$ and the contraction of ${\displaystyle x}$ by ${\displaystyle y}$ is zero.

## C

• Codimension – The codimension of a submanifold is the dimension of the ambient space minus the dimension of the submanifold.

## D

• Diffeomorphism – Given two differentiable manifolds ${\displaystyle M}$ and ${\displaystyle N}$, a bijective map ${\displaystyle f}$ from ${\displaystyle M}$ to ${\displaystyle N}$ is called a diffeomorphism – if both ${\displaystyle f:M\to N}$ and its inverse ${\displaystyle f^{-1}:N\to M}$ are smooth functions.
• Doubling – Given a manifold ${\displaystyle M}$ with boundary, doubling is taking two copies of ${\displaystyle M}$ and identifying their boundaries. As the result we get a manifold without boundary.

## F

• Fiber – In a fiber bundle, ${\displaystyle \pi :E\to B}$ the preimage ${\displaystyle \pi ^{-1}(x)}$ of a point ${\displaystyle x}$ in the base ${\displaystyle B}$ is called the fiber over ${\displaystyle x}$, often denoted ${\displaystyle E_{x))$.
• Frame bundle – the principal bundle of frames on a smooth manifold.

## H

• Hypersurface – A hypersurface is a submanifold of codimension one.

## M

• Manifold – A topological manifold is a locally Euclidean Hausdorff space. (In Wikipedia, a manifold need not be paracompact or second-countable.) A ${\displaystyle C^{k))$ manifold is a differentiable manifold whose chart overlap functions are k times continuously differentiable. A ${\displaystyle C^{\infty ))$ or smooth manifold is a differentiable manifold whose chart overlap functions are infinitely continuously differentiable.

## N

• Neat submanifold – A submanifold whose boundary equals its intersection with the boundary of the manifold into which it is embedded.

## P

• Parallelizable – A smooth manifold is parallelizable if it admits a smooth global frame. This is equivalent to the tangent bundle being trivial.
• Principal bundle – A principal bundle is a fiber bundle ${\displaystyle P\to B}$ together with an action on ${\displaystyle P}$ by a Lie group ${\displaystyle G}$ that preserves the fibers of ${\displaystyle P}$ and acts simply transitively on those fibers.

## S

• Submanifold – the image of a smooth embedding of a manifold.
• Surface – a two-dimensional manifold or submanifold.
• Systole – least length of a noncontractible loop.

## T

• Tangent bundle – the vector bundle of tangent spaces on a differentiable manifold.
• Tangent field – a section of the tangent bundle. Also called a vector field.
• Transversality – Two submanifolds ${\displaystyle M}$ and ${\displaystyle N}$ intersect transversally if at each point of intersection p their tangent spaces ${\displaystyle T_{p}(M)}$ and ${\displaystyle T_{p}(N)}$ generate the whole tangent space at p of the total manifold.
• Trivialization

## V

• Vector bundle – a fiber bundle whose fibers are vector spaces and whose transition functions are linear maps.
• Vector field – a section of a vector bundle. More specifically, a vector field can mean a section of the tangent bundle.

## W

• Whitney sum – A Whitney sum is an analog of the direct product for vector bundles. Given two vector bundles ${\displaystyle \alpha }$ and ${\displaystyle \beta }$ over the same base ${\displaystyle B}$ their cartesian product is a vector bundle over ${\displaystyle B\times B}$. The diagonal map ${\displaystyle B\to B\times B}$ induces a vector bundle over ${\displaystyle B}$ called the Whitney sum of these vector bundles and denoted by ${\displaystyle \alpha \oplus \beta }$.