In nuclear physics, the term refers to the threshold amount (note I do not say 'mass') of a given substance that will cause a chain reaction in that substance. The chain reaction effect is what makes nuclear energy economical, as otherwise splitting atoms would be as hard and expensive as fusing them currently is. (It is the lack of a chain-reaction type effect that is keeping fusion from being an economical energy source - if only a fusion reaction could be made to emit more muons than it consumes...)

Critical mass of a given element or, more usually, isotope is somewhat of a misnomer, since criticality depends upon shape as well as mass. A thin foil of any isotope, no matter how large and heavy, will not become critical, while a sphere of the same mass might instantly detonate. This is because a chain reaction depends upon the neutrons released when an atom fissions striking other atoms and causing them to fission and release more neutrons. If an otherwise critical mass is the wrong shape, too many neutrons will escape into the air (or whatever is surrounding the mass) to cause a chain reaction.

Critical mass for a substance can be reduced by enriching the percentage of isotopes or elements that release lots of neutrons or fission readily. In uranium, critical mass is reduced by enriching the percentage of the 235U isotope, which fissions more readily than the 238U isotope (which tends to absorb neutrons without fissioning). In plutonium, however, each fission releases more neutrons than in uranium, so the critical mass for plutonium is lower - about 2kg, compared to 5kg for weapons grade uranium.

Critical mass is also reduced by the use of shields or 'tamers' that reflect escaping neutrons back into the mass. This means that a subcritical mass of some substance can be triggered into criticality simply by dropping a shield over it, in some cases. Accidents have been reported where researchers have accidentally caused chain reactions by dropping shields, immersing masses in water or concentrating solutions, and in at least one case simply by physical proximity of their bodies acting as reflectors.