That possible early period, beginning at some stated time, during which the failure rate is decreasing rapidly.
Figure 1 - Failure rate / time pattern.
Reference: I.E.C. (modified)
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Celsius (previously Centigrade)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM