Generalized Global Symmetries by Davide Gaiotto and others (2015, 1412.5148, 10.1007/JHEP02(2015)172)

Primary research paper. Introduction, printed pp. 2–3, equations (1.1)–(1.2), defines codimension q+1 symmetry generators and q-dimensional charged operators. A one-form symmetry therefore has codimension-two generators.