what exactly are resonances? are they particles, energy,..?
In what context?
A reasonable description would be particles or particle systems in an excited (unstable) energy state.
One can think of baryonic resonances - Lambda, Sigma, Delta, Xi, Omega particles - http://hyperphysics.phy-astr.gsu.edu/hbase/particles/baryon.html
Or nuclear resonances with respect to neutron absorption cross-seciton.
Separate names with a comma.