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Reform means a useful change, or sometimes to a pure original state.
Reform is generally distinguished from revolution. Revolution means basic or radical change; reform may be no more than fine tuning, or the removing of serious wrongs without changing the whole system. Reform seeks to improve the system as it stands, never to overthrow it.
A note about grammar: when used to describe something which is physically formed again, such as re-casting it in a mold/mould, or a band that gets back together, the proper term is re-form (with a hyphen), not "reform".