Quasisymmetric (quasihelically symmetric) plasma equilibria are non-axisymmetric configurations in which the magnetic field strength depends only on one angular coordinate within the magnetic flux surfaces.  This concept is part of the program of stellarator optimization (designing stellarators to have reduced transport, i.e., heat and particle losses). 
Types of quasisymmetry:
- Quasihelical (QH) symmetry, on which the design of the HSX stellarator (operational) is based.
- Quasipoloidal (QP) symmetry, on which the design of the QPS stellarator (under construction) is based.
- Quasi-axisymmetry (QA), on which the design of the NCSX stellarator was based
- D.A. Garren and A.H. Boozer, Existence of quasihelically symmetric stellarators, Phys. Fluids B 3 (1991) 2822
- Iván Calvo et al, Stellarators close to quasisymmetry, Plasma Phys. Control. Fusion 55 (2013) 125014
- J.N. Talmadge, F.S.B. Anderson, D.T. Anderson, C. Deng, W. Guttenfelder, K.M. Likin, J. Lore, J.C. Schmitt, K. Zhai, Experimental Tests of Quasisymmetry in HSX, Plasma and Fusion Research 3 (2008) S1002
- D.A. Spong, S.P. Hirshman, J.F. Lyon, L.A. Berry and D.J. Strickler, Recent advances in quasi-poloidal stellarator physics issues, Nucl. Fusion 45 (2005) 918
- M.Yu. Isaev et al, The pseudo-symmetric optimization of the National Compact Stellarator Experiment, Phys. Plasmas 6, 8 (1999) 3174