Acidizing and hydraulic fracturing are the two common kinds. Both change the near-well flow path. Neither changes the oil or gas in place, and a forecast that treats a stimulated rate as if the reservoir had grown will overstate the recovery.
A stimulation is judged by the productivity index, or by the skin, before and after, at comparable reservoir pressure. A rate that is higher only because the well was shut in and the pressure built up is not a stimulation result.