A transmutation is a process in which an atomic nucleus goes from one position in the periodic table to another, usually one up or one down.
For example, potassium occupy the 19th position in the periodic table while calcium occupies the 20th position. The two elements are separated by a single charge. If a potassium nucleus drops a negative charge, as would be the case if it sheds an electron, it would no longer have a net charge of 19 but 20. It would have moved one up in the periodic table to become calcium.
Conversely, if a calcium nucleus consumes an electron, its net charge will go down from 20 to 19. This would move it one position down in the periodic table. It would be transmuted to potassium.
The mass of the atom before and after such a transmutation would be almost the same. They would only differ in mass by a single electron. This means that potassium would become a light isotope of calcium. Regular calcium would likewise become a heavy isotope of potassium.
All of this can be understood in terms of the proton-electron atom.
With the assumption that light is dielectric matter, a very similar mechanism can be added to this story. Since a photon has the potential of becoming an electron-positron pair, we can in theory transmute matter up and down the periodic table by the use of light.
An atomic nucleus that consumes a positron without also consuming an electron will go one up in the periodic table in the same way that an atomic nucleus that consumes an electron without also consuming a positron goes down in the periodic table.
Potassium can in this way be transmuted into calcium. A high energy photon breaks into an electron-positron pair. The potassium nucleus consumes the positron, while letting the electron escape.
The calcium produced in this way will be heavier than the original potassium by the weight of a single positron. This kind of transmutation is therefore a type of mass condensation.
We also have the possibility of atomic nuclei transmuting through the consumption or shedding of a proton. This kind of transmutation is what we normally refer to as nuclear fusion and fission.
Potassium can be transmuted into calcium through fusion with a proton, and calcium can be transmuted into potassium through fission.
Any kind of fusion or fission can be considered a type of transmutation. One such transmutation is the Sodium to Potassium transmutation that some believe our bodies to be capable of performing.