To help clear up any confusion regarding what your eyeglasses prescription really means, it is important to understand a few basic words and abbreviations that can be found on the prescription. The two most important of these mystery terms and abbreviations are OD (Oculus Dextrus) and OS (Oculus Sinister). These are Latin terms that mean right eye and left eye, respectively. Listed after these OD and OS abbreviations are numbers preceded by either a plus sign (+) or a minus sign (-). A plus sign indicates that the designated eye is farsighted, while a minus sign indicates that the designated eye is nearsighted. This is measured in diopters, the unit of measurement that indicates the nearsightedness or farsightedness of the eye. Diopters are normally abbreviated to a simple D, so an OD -1.25 indicates that a patient is one and one quarter of a diopter farsighted in their right eye.Astigmatism might also appear on an eyeglasses prescription for a person. Prescriptions for astigmatism are written in a certain format, S x C x Axis. The S stands for spherical. This is how round the eye is. This can be a positive or negative number. The C stands for cylinder and is another word for astigmatism. This can also be a positive or negative number, and measures the amount of astigmatism in diopters. The larger the diopter number, the more of an astigmatism the patient has. The Axis is the last number in a formula for astigmatism. This is a number anywhere between 0 and 180, and is measured in degrees. The axis denotes the position of the astigmatism as far as where it is taking place in the curve of the eye. This is critical in ensuring the glasses are made properly.