epsilon ijk

The title of the node is mean to read ε_ijk, but node titles cannot contain subscripts.
Anyway, ε_ijk is a third rank tensor on R³, which takes the value 1 when (ijk) is an even permutation of (123) (ie. (123),(231),(312)), the value -1 if (ijk) is an odd permutation of (123) (ie. (321),(213),(321)), and is zero if otherwise.
It is used for expression vector products in abstract index notation, eg a=b×c becomes a_i=ε_ijkb_jc_k, similarly for curl b if you replace a by nabla and a_j by partial-d-by-dx_j.
You can also write the determinant of a 3×3 matrix A as ε_ijkA_1iA_2jA_3k