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.