1) Rydberg constant 2) Rydberg unit 3) Wave number 
[crater] This is a little-eroded crater formation with a relatively sharp edge. The inner walls do not have much terracing, although slumped piles of talus lie along the base. At the midpoint of the interior is a central ridge, with a low spur that runs halfway toward the southern inner wall. This feature was named after the Swedish physici...
[n] - a wave number characteristic of the wave spectrum of each element
rydberg constant noun a wave number characteristic of the wave spectrum of each element
a unit of energy used in atomic physics, equal to 13.606 electron-volts. Abbr.: ry