A mineral reserve is the part of a resource that a study has shown can be mined at a profit under stated assumptions.
The economically mineable portion of a measured or indicated mineral resource, demonstrated by at least a preliminary feasibility study. Mineral reserves include consideration of mining, metallurgical, economic, marketing, legal, and environmental factors.
Reserves are the closest thing in mining disclosure to a commercial commitment. Declaring one means a qualified person has applied real costs, real recoveries and a stated metal price and concluded the material pays for its own extraction. Very few juniors ever get there.
Reserves come in two categories, proven and probable, derived from measured and indicated resources respectively. The metal price assumption sits in the technical report and is worth finding — a reserve declared at $2,400 gold behaves differently at $1,700.
Reserves are always smaller than the resource they come from, sometimes dramatically. A company quoting only its resource may be doing so because the reserve is unflattering or does not exist.
108 terms have their own page: the definition, why it matters to an investor, where it shows up in practice, and the mistake people make with it.