Proportion:

A statement that two ratios are equal is called a proportion.eg: 

3:5 = 6:10 it means 3:5 :: 6:10( read as ‘a is to b as c is to d)

In the proportion a:b :: c:d , a,b,c,d are called first, second, third and fourth terms respectively. 

First and fourth terms of a proportion are called extreme terms or extremes

The second and third terms are called middle terms or means.

Product of extremes = Product of means