1) An abnormally high fever 2) Extremely high fever 3) Febricity 4) Febrility 5) Fever 6) Feverishness 7) Hyperthermia 8) Hyperthermy 9) Pyrexia 
excessive body temperature
• (n.) A condition of excessive fever; an elevation of temperature in a disease, in excess of the limit usually observed in that disease.
exceptionally high fever either in comparison of the fever usually accompanying a particular disease or absolutely (as in heat stroke).
A very high fever, at a level that is likely to cause damage to tissue. 
<clinical sign> Exceptionally high fever either in comparison of the fever usually accompanying a particular disease or absolutely (as in heatstroke). ... (18 Nov 1997) ... 
(hi″pәr-pi-rek´se-ә) hyperthermia. adj., hyperpyrex´ial, hyperpyrex´ic., adj. malignant hyperpyrexia malignant hyperthermia.
Hy`per·py·rex'i·a noun [ New Latin See Hyper- , and Pyrexia .] (Medicine) A condition of excessive fever; an elevation of temperature in a disease, in excess of the limit usually observed in that disease. 
Type: Term Pronunciation: hī′pĕr-pī-rek′sē-ă Definitions: 1. Extremely high fever.
[n] - extremely high fever (especially in children)
hyperpyrexia (high' pur pigh REK see uh) An abnormally high fever, either in comparison to the normal body temperature of a specific disease or as in a heat stroke.
an abnormally high fever.
[Difficult words] extremely high fever (especially in children)[Obscure words] extremely high fever (especially in children)