Is it two-third or two-thirds? If both are correct, how does their usage differ?
A third, like any other unit of measurement, is dependent on the number of units described. For example, we describe 'one apple' and 'two apples'. The same goes for fractional units. 'one third' is simply a single unit of 'third'. Two or more and we use 'thirds'; hence, 'two-thirds' is the correct usage.