Acute describes a disease or condition that comes on suddenly and runs a short course. In clinical use it also implies greater severity than the baseline — an acute episode is more intense, more urgent, or more dangerous than the same condition in its mild or chronic form.
The opposite is chronic, which refers to a condition that develops slowly or persists over a long time. A horse may have a chronic lameness that suddenly worsens into an acute episode — same underlying problem, different clinical picture. The distinction matters for triage: acute presentations typically require immediate veterinary attention, while chronic conditions are managed over time.
The word appears throughout equine medicine: acute colic, acute laminitis, acute respiratory distress. In each case it signals that the onset was rapid and that the situation warrants immediate attention. A hoof abscess presents acutely — severe sudden lameness — though its underlying cause may have been building for weeks. Recognizing the acute phase early is a core skill for any horse owner.
Further reading: Acute (medicine) on Wikipedia; lameness in horses from the Merck Veterinary Manual, a prototypical acute equine presentation.