Sentence:  The dawn of the oil <WSD>age</WSD> was fairly recent. Although the stuff was used to waterproof boats in the Middle East 6,000 years ago, extracting it in earnest began only in 1859 after an oil strike in Pennsylvania. The first barrels of crude fetched $18 (around $450 at today's prices). It was used to make kerosene, the main fuel for artificial lighting after overfishing led to a shortage of whale blubber. 
Meaning of the word marked with <WSD> tags:  A particular period of time in history, as distinguished from others.