Boil, seethe, simmer, parboil, stew mean to prepare (as food) in a liquid heated to the point where it emits considerable steam.
Boil implies the bubbling of the liquid and the rapid escape of steam; it may be applied to the liquid alone, but usually it suggests a fast method of accomplishing an end (as cooking or cleansing).
Seethe differs only slightly from boil. It emphasizes the subjection of something to the influence of a boiling liquid in order to cook it thoroughly or to make an infusion of it.
This difference, though slight, is also apparent in extended senses of boil and of seethe, for boil suggests a sudden rise and ebullition and seethe suggests the agitation and turmoil which follows a cause of excitement.
Simmer suggests that the liquid is at the point of boiling; it implies less steam and less bubbling than boil and is used, therefore, to denote a gentle and slower form of cooking.
Parboil usually implies boiling for a limited time to prepare some food for further cooking (as by roasting or frying).
Stew implies long slow simmering, usually in a closed vessel; it is used especially in reference to meats or fruits cooked until they are tender or broken up.