Determine the next term in the following number series: 3, 15, 35, 63, ?

The next term is 99.

Each term is a product of two consecutive odd numbers.

3 = 1 x 3, 15 = 3 x 5, 35 = 5 x 7, 63 = 7 x 9.

The next pair is 9 x 11.

So the term = 9 x 11 = 99.