Deconfinement is the transition by which quarks and gluons, normally locked inside color-singlet bound states, become liberated in a hot and/or dense medium to form the quark-gluon plasma. It occurs in QCD around \(T_c\approx 155\) MeV (a crossover at zero baryon density).

• Polyakov loop \(\langle L\rangle\): related to the free energy \(F_q\) of a single static quark via \(\langle L\rangle\sim e^{-F_q/T}\). In the confined phase \(F_q\to\infty\) so \(\langle L\rangle=0\); in the deconfined phase \(\langle L\rangle\neq0\). For pure gauge theory it is an exact order parameter of the \(Z_{N_c}\) center symmetry.
• Temporal string tension \(\sigma(T)\): non-zero (confining, linear potential) below \(T_c\), and vanishes above \(T_c\) because color charges are Debye-screened (see Debye mass).

Deconfinement screens the electric sector: the chromoelectric field of a static charge is Yukawa-suppressed over \(1/m_D\sim1/(gT)\). The magnetic sector, however, is *not* screened at leading order. Consequently a purely spatial Wilson loop still obeys an area law: the spatial string tension \(\sigma_s(T)\) stays finite and even grows across and above \(T_c\). This "magnetic confinement" is captured by the reduced 3D theory MQCD and is the observable studied in Why spatial string tension and dimensional reduction.