John C. Baez and James Dolan?, Higher-Dimensional Algebra III. n-Categories and the Algebra of Opetopes, Advances in Mathematics 135, pages 145-206, 1998. arXiv:9702014

Idea

This paper introduces opetopes?, and uses them to give a definition of weak n-categories?. The term 2-rig was first used here.

References

P. Gabriel? and M. Zisman?, Calculus of Fractions and Homotopy Theory?, Ergebnisse der Math. und ihrer Grenzgebiete_, vol. 35, Springer, Berlin, 1967.