A * bundle* P = (P, M ,π; G) is a principal bundle if the standard fiber is a Lie group G and ∃ (at least) one trivialization the transition functions of which act on G by left translations Lg : G → G : h ↦ f g . h (where . denotes here the group multiplication).

The principal bundles are slightly different from affine bundles and vector bundles. In fact, while in affine bundles the fibers π^{-1}(x) have a canonical structure of affine spaces and in vector bundles the fibers π^{-1}(x) have a canonical structure of vector spaces, in principal bundles the fibers have no canonical Lie group structure. This is due to the fact that, while in affine bundles transition functions act by means of affine transformations and in vector bundles transition functions act by means of linear transformations, in principal bundles transition functions act by means of left translations which are not group automorphisms. Thus the fibers of a principal bundle do not carry a canonical group structure, but rather many non-canonical (trivialization-depending) group structures. In the fibers of a vector bundle there exists a preferred element (the “zero”) the definition of which does not depend on the local trivialization. On the contrary, in the fibers of a principal bundle there is no preferred point which is fixed by transition functions to be selected as an identity. Thus, while in affine bundles affine morphisms are those which preserve the affine structure of the fibers and in vector bundles linear morphisms are the ones which preserve the linear structure of the fibers, in a principal bundle P = (P, M, π; G) principal morphisms preserve instead a structure, the right action of G on P.

Let P = (P, M, π; G) be a principal bundle and {(U_{α}, t_{(α)}}_{α∈I} a trivialization. We can locally consider the maps

R^{(α)}_{g} : π^{-1}(U_{α}) → π^{-1}(U_{α}) : [x, h]_{(α)} ↦ [x, h . g]_{(α)} —– (1)

∃ a (global) right action R_{g} of G on P which is free, vertical and transitive on fibers; the local expression in the given trivialization of this action is given by R^{(α)}_{g} .

Using the local trivialization, we set p = [x, h]_{(α)} = [x, g_{(βα)}(x) . h]_{β} following diagram commutes:

which clearly shows that the local expressions agree on the overlaps U_{αβ}, to define a right action. This is obviously a vertical action; it is free because of the following:

R_{g}p = p => [x, h . g]_{(α)} = [x, h]_{(α)} => h · g = h => g = e —– (2)

Finally, if p = [x, h_{1}]_{(α)} and q = [x, h_{2}]_{(α)} are two points in the same fiber of p, one can choose g = h_{2}^{-1} . h_{1} (where · denotes the group multiplication) so that p = R_{g}q. This shows that the right action is also transitive on the fibers.

On the contrary, that a global left action cannot be defined by using the local maps

L^{(α)}_{g} : π^{-1}(U_{α}) → π^{-1}(U_{α}) : [x, h]_{(α)} ↦ [x, g . h]_{(α)} —– (3)

since these local maps do not satisfy a compatibility condition analogous to the condition of the commuting diagram.

let P = (P, M, π; G) and P’ = (P’, M’, π’ ; G’ ) be two principal bundles and θ : G → G’ be a homomorphism of Lie groups. A bundle morphism Φ = (Φ, φ) : P → P’ is a principal morphism with respect to θ if the following diagram is commutative:

When G = G’ and θ = id_{G} we just say that Φ is a principal morphism.

A trivial principal bundle (M x G, M, π; G) naturally admits the global unity section I ∈ Γ(M x G), defined with respect to a global trivialization, I : x ↦ (x, e), e being the unit element of G. Also, principal bundles allow global sections iff they are trivial. In fact, on principal bundles there is a canonical correspondence between local sections and local trivializations, due to the presence of the global right action.