"ism" usually referrs to a philosophy, religion or practice. People using this word most likely mean the study and use of herbs. Herbs can be used for many things from medicine, cleaners, dyes etc. and have magical applications as well.
To me it means the study of herbs for a physical perspective. Although many people use them in Magical workings, I prefer to limit my knowledge to what can interact with the human body. As such I tend to be stricter in what I read in regards to learning herbalism and remedies they provide.