Words are the medium by which we communicate our ideas; indeed, they are the tools of thought itself. Realizing that clear, precise, and accurate thinking, speaking, and writing require clearness, precision, and accuracy in the use of words, we have undertaken to clarify some of the ambiguity that prevails in the field of ocular motility and allied sub jects. The first essential for any science is to agree on a terminology with exact definitions in order that we may speak the same language and write in exact terms concerning it. We realize that it is impossible to bring about unanimous agreement, that many writers and teach ers will continue to use terms with their own private preferred meanings, regard less of what other writers have done and are doing. Many have given the matter no careful thought but have followed the usage of those with whom they have come in contact, even if their leadership was unreliable. But we are hopeful that many will welcome an attempt, especially if it is based on a study of the usage of the best the most worthy lead ers (meaning those who show that they give careful thought to the terms they use, that they are well acquainted with the literature and therefore with the usage of previous writers, and who further show a clarity of expression in their writ ings which is the fruit of clear and precise thinking). Of course we do not claim to speak with authority. Our decisions in dubious cases can carry weight only in so far as we present convincing reasons for our opinions. It is better to have uniformity and definiteness, even if there is not in fallibility. We invite constructive criticism and expression of opinion, especially if ac companied with reasons. The reader will look in vain in this list for terms that are well established and about which there is no uncertainty, no ambiguous usage. These we take for granted. The terms we do include are grouped under the headings : points, lines, planes, fields, angles, units, movements, fusion, stereopsis, heterophoria, heterotropia, abbreviations, words misspelled, mispronounced, words in the choice of which good writers differ.