"A radiation induced health effect that occurs within months of the exposure that caused it. Generally, a threshold level of dose exists below which the effect is not observed. Above the threshold, the severity of the effect increases with dose. Examples of early effects include erythema (burning of the skin) and acute radiation syndrome (radiation sickness with vomiting and diarrhoea)."
http://www.rpii.ie/Site/Education/Glossary.aspx#s1458
http://www.rpii.ie/Site/Education/Glossary.aspx#s1458
