Vitamers represent:
Vitamers:
The term vitamers represents the chemically similar substances that possess qualitatively similar vitamin activity. 
Some good examples of vitamers are given below:

Retinol, retinal and retinoic acid are vitamers of vitamin A.
Pyridoxine, pyridoxal and pyridoxamine are vitamers of vitamin B6.

Ref :  Textbook of Biochemistry, Satyanarayana
Chemically similar  substances that  possess qualitatively similar vitamin  activity
Chemically dissimilar  substances that  possess qualitatively similar vitamin  activity
Chemically similar  substances that  possess qualitatively less vitamin  activity
Chemically similar  substances that  possess qualitatively more vitamin  activity