A plasmon is a quantum of plasma oscillation, which occurs when free electrons in a material, such as a metal, collectively oscillate in response to an external electromagnetic field. This phenomenon is significant in the study of nanotechnology and optics, as it can enhance light-matter interactions at the nanoscale. Plasmons can be classified into two main types: bulk plasmons, which propagate through the volume of a material, and surface plasmons, which exist at the interface between a metal and a dielectric. Surface plasmons are particularly important in applications like surface-enhanced Raman scattering (SERS) and plasmonic sensors, where they can amplify signals and improve detection sensitivity.