An irrotational vector field is a vector field where the curl of the vector field is zero everywhere in the region considered. This means that the field has no local rotation or 'twisting' around any point, which implies that the field can be expressed as the gradient of a scalar potential function. In the context of line integrals, if a vector field is irrotational, it has certain properties that simplify calculations and can lead to path independence in the evaluation of line integrals.