Perimeter: A perimeter is one of the outline loops an FDM printer traces around each layer to form a part's walls, set in the slicer as a wall count.
In each layer of an FDM print the nozzle traces the outline of the part, then one or more loops inside it, then fills the rest with infill. Those loops are the perimeters. With a 0.4 mm nozzle, three perimeters make a wall about 1.2 mm thick. Top and bottom skins do the same job on horizontal faces.
What it means for the price
Perimeters are printed slowly and carefully, the outer one most of all, because they form the visible surface. A part with a large surface area and little volume, such as a thin shell or an enclosure, spends most of its print time on perimeters, so its time follows surface area more than volume. That is why a print time estimate built from volume alone goes wrong on thin parts, and why a good estimate uses both.
Example: on a thin-walled cylinder 80 mm across, going from 2 to 4 perimeters can add more time than raising infill from 20% to 50%, because the infill region there is small. The figures are an example.
Common confusions
Slicers call perimeters walls, shells or wall loops. A wall count in the slicer is not the wall thickness in the CAD model: a modelled wall thinner than two perimeters prints poorly or not at all, which is what a minimum wall thickness check catches. Read how FDM print time is estimated.
Go deeper: FDM print time estimation: why volume does not predict it