Noble metals are metals that are resistant to corrosion or oxidation, unlike most base metals. They tend to be very valuable, often due to perceived rarity. Examples include gold, silver, tantalum, platinum, and palladium.
Some of the noble metals can be dissolved in aqua regia, a highly concetrated mixture of acids.
Alchemists were concerned with the transmutation of base metals into gold (a noble metal) for economic gain, or as a metaphor for more esoteric processes.
The term can also be used in a relative sense. A "Galvanic series" is a hierarchy of metals (or other electrically conductive materials, including composites and semimetals) that runs from noble to active, and allows designers to see at a glance how materials will interact in the environment used to generate the series. In this sense of the word, graphite is more noble than silver (even though it is alchemically more base) and the relative nobility of many materials is highly dependent upon context, as for aluminium and stainless steel in conditions of varying pH.
See also