Flux
On the theoretical chemistry level, a flux is an oxide that lowers the melting or softening temperature of a mix of others. Fluxing is about interaction, temperature range and relative amounts: when both the flux and the oxide mix with which it interacts are present in the correct proportions in the right temperature range, the melting temperature of the mix is much lower than either of the two separately. Fluxing can also be about atmosphere, iron for example, is a strong flux in reduction yet very refractory in oxidation. Examples of fluxing oxides for high temperature glazes are K2O, Na2O, CaO, Li2O, MgO, ZnO (CaO and MgO are not active at lower temperatures). In glaze chemistry, each of these oxides is an individual with its own optimal percentage and interaction with silica and alumina. Fluxing oxides make up a minor part of the glaze, they interact with the SiO2 glass former and Al2O3 (and other fluxes). If used in this way, CaO, for example, reacts strongly with stoneware and porcelain glazes to lower their melting temperature. But this same oxide, as the material calcium carbonate, is refractory in a 75:25 mix with bentonite (where the conditions for interaction to produce a glass are not present).
Colorants can also be powerful fluxes. Copper, cobalt and manganese all melt very actively in oxidation and reduction, whereas iron is very active in reduction.
When the term flux is used on the material level, it is referring to the fact that the chemistry of the material contributes a significant amount of one or more of the fluxing oxides. These materials do not necessarily melt well by themselves. Dolomite, like calcium carbonate, is a stoneware glaze flux. But it does not melt by itself (it can be dead-burned and used as a heavy duty refractory for ladles and slag furnaces). Talc in small percentages in middle temperature clay bodies acts as a strong flux, whereas in large percentages it is refractory also.
B2O3 is a very low melting oxide, the ceramic industry depends very heavily on it. But B2O3 is not a flux, it is a low melting glass (it does not depend on percentage and interaction to activate, it works across the entire temperature range used in traditional ceramics). Almost all frits contain at least some B2O3.
Fluxing oxides in frits melt much better than in raw materials. MgO is an excellent example. Glazes that employ frit to supply the MgO melt much better than those employing dolomite or talc.
Understandably, predicting the effects of a flux addition to a glaze (e.g. melting temperature) is very complex (involving interactions, eutectics, proportions, premelting and the physical and mineralogical properties of the particles). For this reason, ceramic chemistry is applied much more in a relative sense than absolute.
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Pictures
At cone 10 CaO is a powerful flux. 10% calcium carbonate added to the under-melted bubble-laden celadon on the left transforms it into an fluid super-gloss with no bubbles.
Example of various materials mixed 75:25 with volclay 325 bentonite and fired to cone 9. Plasticities and dry shrinakge vary widely. Materials normally acting as fluxes are refractory.
1215U flow test, MgO is sourced from Talc (right) and from a much more actively melting MgO frit (left).
Metallic oxides with 50% Ferro frit 3134 in crucibles at cone 6ox. Chrome and rutile have not melted, copper and cobalt are extremely active melters. Cobalt and copper have crystallized during cooling, manganese has formed an iridescent glass.
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