A Modern Dictionary of Geography 4th Edition is a must have for all students of any form of Geography.
The dictionary is by three British professors and it gives academic but straightforward and easily understandable definitions. The book is surprisingly free of political overtones (something which British geography is cursed with). I do think it favors physical geography over the human and technical subfields; however, this does not negate its essentialness to human geographers. On a very plus note it has easy to understand quantitative formulas and explanations of the formulas.
Catholicgauze gives an A+