Plain English Meaning
Double vision. You look at one thing and see two of it because your eyes aren't pointing at exactly the same spot.
Full Definition
A visual disturbance in which a single object is perceived as two overlapping or side-by-side images, caused by the eyes failing to align properly on the same point. In aviation, diplopia can result from hypoxia, decompression sickness affecting the central nervous system, fatigue, or certain medications.
Context Anchor
Seen in lists of possible altitude-induced decompression sickness symptoms after flight at high altitude or after a rapid pressure change.
Derivation
From the Greek 'diplous' meaning 'double' and 'ops' meaning 'eye.' Literally 'double-eye' — a useful anchor because the symptom is exactly that: seeing double.