Sentence:  Since the launch early last year of [...] two Silicon Valley start-ups offering free education through MOOCs, massive open online courses, the ivory towers of academia have been shaken to their foundations. University brands built in some cases over <WSD>centuries</WSD> have been forced to contemplate the possibility that information technology will rapidly make their existing business model obsolete. 
Meaning of the word marked with <WSD> tags:  A period of 100 consecutive years; "often specifically" a numbered period with conventional start and end dates, e.g., the "twentieth century", which stretches from ("strictly") 1901 through 2000, or ("informally") 1900 through 1999. The "first century AD" was from 1 to 100.