Official WSSA definitions of "Herbicide Resistance" and "Herbicide Tolerance"

Herbicide Resistance : "Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type.  In a plant, resistance may be naturally occurring or induced by such techniques as genetic engineering or selection of variants produced by tissue culture or mutagenesis."

Herbicide Tolerance : "Herbicide tolerance is the inherent ability of a species to survive and reproduce after herbicide treatment.  This implies that there was no selection or genetic manipulation to make the plant tolerant; it is naturally tolerant."