Imino acid

Imino acids are a group of compounds that contain both an amide and a carboxyl group, bonded to the alpha carbon molecule ^[1]. The difference between amino acids and imino acids is in the bonding of the nitrogen in the amide group. In imino acids, the nitrgen forms a double covalent bond to another molecule, or two single bonds to two different 'R' groups.

Proline

Although Proline is often refered to as an amino acids, it has a secondary amide group, making it an imino acid ^[2]. This is due to its cyclic nature, as the nitrogen in the amide group is bonded to two different 'R' groups in the ring. 

References:

1. IUPAC, Compendium of Chemical Terminology, 2nd ed. (the "Gold Book") (1997). Online corrected version: (2006–) "Imino acids". (http://goldbook.iupac.org/html/I/I02959.html)

2. NCBI, 'Proline. The Lord of the Rings'. Viewed on 03/12/2017 (https://www.ncbi.nlm.nih.gov/Class/Structure/aa/aa_explorer.cgi?mode=compare&res1=P&format=text)