Alum

Alum is both a specific chemical compound and a class of chemical compounds.

It means potassium aluminium sulfate, a chemical compound. 
It also means a double sulfate salt.

Examples:
Potash alum is potassium aluminum sulfate
Sodia alum is sodium aluminum sulfate
Ammonium alum is ammonium aluminum sulfate

 History 
Words like "alum" and "potash" were used before modern chemistry developed. The word 'alumen' occurs in Pliny's Natural History (2379 AD). In the 52nd chapter of his 35th book, he gives a detailed description. Different substances were distinguished by the name of 'alumen', but they were all astringents,Astringent: causes body tissue to shrink or contract. and were all used in dyeing and medicine.

 Notes 

Category:Sulfur compounds