In organic chemistry, antarafacial (Woodward-Hoffmann symbol a) and suprafacial (s) are two topological concepts in organic chemistry describing the relationship between two simultaneous chemical bond making and/or breaking processes in or around a reaction center.
[1] The reaction center can be a p- or spn-orbital (Woodward-Hoffmann symbol ω), a conjugated system (π) or even a sigma bond (σ).
The suprafacial shift is symmetry-forbidden because orbitals with opposite algebraic signs overlap.
The symmetry allowed antarafacial shift would require a strained transition state and is also unlikely.
In contrast a symmetry allowed and suprafacial [1,5]-hydride shift is a common event.