In the absence of external perturbations, a molecule normally exists in its most stable condition, called the Ground Electronic State.
When the molecule takes up more energy than usual, it enters a so called Excited Electronic State. There are many different excited electronic states, depending on the amount of energy absorbed by the molecule. The change from the ground to an excited state is called Electronic Excitation.
This process can be illustrated as in the figure below where the chemical structure of the thymine molecule, one of the four DNA bases, is shown.
To better understand this process, and what an Excited Electronic State really is, scientists use the concept of electronic orbitals. The electrons of atoms move very rapidly within a more a less defined space around the nucleus, called Atomic Orbitals. In the case of molecules, the term of Molecular Orbitals is used.