Terms in a Proportion are of the following types:
1) First Term
2) Second Term
3) Third Term
4) Fourth Term
5) Extreme Terms
6) Middle Terms
e.g. In a statement of Proportion :
21 : 9 :: 28 : 12
First Term = 21
Second Term = 9
Third Term = 28
Fourth Terms = 12
Extreme Terms = 21 & 12
Middle Terms = 9 & 28
Explanation of these Terms with more examples is available in the above links
Note = When two Ratios are in Proportion, Product of Extreme Terms is always equal to Product of Middle Term

