fecula is a general term for a starch which has been rendered more or less pure by washing in water, as opposed to a flour or farina, which contains protein and other substances from the original plant. Most ordinary starches such as tapioca and cornflour are feculas. So is ‘ground rice’, which is purified rice starch and is more accurately called crème de riz by the French. semolina, on the other hand, contains much protein.
Some French recipes call for ‘fécule’, e.g. to thicken soups. This is understood to mean potato starch, which is available everywhere in France and in some shops in other countries. It is effective in very low concentrations.