The M-unit is a standardized unit used to specify the size of printed text or optotypes for visual acuity testing independent of testing distance. By definition, the detail of 1M print subtends a visual angle of 1 minute of arc (1/60 of a degree) at a viewing distance of 1 meter. The overall 1M letter or optotype subtends 5 minutes of arc at this same 1 meter viewing distance. Likewise, the detail of 2M print subtends 2 minutes of arc at 1 meter and the letter subtends 1o minutes of arc at 1 meter.

An incomitant strabismus resulting from physical impedance of ocular rotations, rather than extraocular muscle paresis. The deviation varies in magnitude in different positions of gaze. Etiologies include extraocular muscle enlargement, as in thyroid eye disease; orbital scarring after trauma or surgery; and anomalous orbital structures.

Monofixation syndrome, alsos known as microtropia, is a small angle strabismus, typically an esotropia, measuring less than 5-8 prism diopters. It is often difficult to detect on cover testing due to the subtle deviation. A central suppression scotoma is usually present in the deviating eye. Microtropia can occur as a primary condition or secondary to treatment of a larger angle strabismus.

Motor fusion, also known as fusional vergence or fusional reserves, is the maximum amount the eyes can converge or diverge while still maintaining binocular single vision. Positive fusional reserves, measured with base-out prisms, represent the eyes’ maximum convergence ability. Negative fusional reserves, measured with base-in prisms, represent maximum divergence.

An extraocular muscle surgery where a segment of the muscle belly is excised to weaken the muscle’s pulling force. It is commonly performed on the inferior oblique muscle to treat persistent inferior oblique overaction.