Debye length is the characteristic distance over which a plasma screens out electric fields — beyond it, the rearrangement of free charges cancels any local charge imbalance and the plasma behaves as electrically neutral.
The scale is named for Peter Debye, who derived it with Erich Hückel in 1923 to describe charge screening in electrolyte solutions; plasma physics adopted the same mathematics. Around any charge embedded in a 📝plasma, mobile electrons shift to form a screening cloud, and the charge's electric field dies away within a few Debye lengths instead of reaching across the volume. The length shrinks as density rises and grows as temperature rises; in a magnetically confined fusion plasma it is measured in tens of micrometers — vanishingly small next to the meters-scale plasma itself.
That separation of scales is what makes the Debye length definitional rather than incidental: an ionized gas qualifies as a plasma only when its dimensions are much larger than its Debye length, so that collective behavior dominates over individual particle interactions. The concept underpins quasineutrality — the working assumption that electron and ion charge densities balance almost exactly everywhere — which is the starting point of nearly every theory used to model confined fusion plasmas, from large-scale stability to turbulence simulation.