Quasiparticle

The term Quasiparticle describes a concept in physics in which energetically excited states in matter are treated as though they are elementary quantum particles in their own right.  In fact they may be something less tangible like a vibration or a rotation.
Unlike neutrons, protons, or even atoms and molecules, quasiparticles aren't independent structures floating about on their own in free space but exist only within the structure being studied. 
For the purposes of modelling the behaviour of the excited state within the structure this treatment works very well.  
As the particles emerge from more fundamental structures they are called quasi-particles or quasiparticles.
These particles have in most cases short  lifetimes, like the magnon or the exciton. But there are also some quasi-particles with an infinite lifetime like the cooper-pair in superconductors.