A unit fraction is a rational number written as a fraction where the numerator is one & the denominator is a positive integer. A unit fraction is therefore the reciprocal of a positive integer. Examples; 1/2, 1/3, 1/4, etc.
a unit fraction is each piece of the fraction. so if you have a rectangle cut into fourth than each piece of that rectangle is 1/4. each 1/4 is a unit fraction. Unit fractions will always have one as a numerator.
