Premature means it happened before it should have. A celebration started before the final whistle; an announcement made the day before the deal was actually signed.
The commonest use by far is about babies: a premature baby is born several weeks early, and hospitals shorten it to premmie or preemie. Beyond the ward it is the word for acting too soon — a premature conclusion, a premature decision, 'it would be premature to comment at this stage'. The adverb is prematurely: prematurely grey, prematurely born. It carries a small judgement inside it; calling something premature says you should have waited. And it is not a neutral word for early — an early train is not a premature train.
1adjectiveprematureHappening or done before the proper time.
“Any announcement now would be premature.”
2adjectivebabyOf a baby: born before the normal length of pregnancy is complete.
“The twins were premature and spent a month in hospital.”
3adjectiveearlierHappening earlier than it should, especially of death or ageing.
“Smoking remains the main cause of premature death in the region.”
Premature is Latin praemātūrus: prae, 'before', joined to mātūrus, 'ripe'. Something premature has been picked before it ripened. The word turns up in English around 1440, in a translation of a Roman farming manual — exactly where a word about unripe things ought to begin.
Latin
praemātūrus
English, today
premature
From Latin praemātūrus, equivalent to pre- + mature. First use appears c. 1440 in a translation of Palladius's De Re Rustica.
Etymology from Wiktionary, CC BY-SA