Molecular Gastronomy is the segment of food science that studies the changes that ingredients experience during cooking. It strives to answer the questions of why foods do the things they do when heated, cooled, stirred, or left alone. At least that's what it meant when the term was coined in the late 1980's.
The term molecular gastronomy has come to be associated with a modern discipline of cooking that uses a set of ingredients that until now were not readily available. The modernist movement uses new techniques to alter the texture of familiar ingredients, and present them in an entirely different way. The modern chef looks to evoke a memory, or take the diner to a time and a place by poking at the sensory memory of taste, sight, and smell. Today's modern kitchen can make a liquid a solid, or a liquid an "air", which is a light and airy foam that dissipates the second it is eaten. The diner is left with just a hint of flavor to accent the main ingredient.
Even though the ground work for this discipline was laid by the scientists that coined the term Molecular Gastronomy, the term itself is a polarizing one. The chefs that embrace these modern techniques prefer "modernist" or "innovative" cuisine. To most people, Molecular Gastronomy today conjures up visions of liquid nitrogen, and obscure chemicals that create a gimmicky form of cooking that looks more like art than dinner. In fact, the products are not chemical at all, but rather all natural derivatives of plants and seaweed.