The title of the node is mean to read εijk, but node titles cannot contain subscripts.
Anyway, εijk 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 aiijkbjck, similarly for curl b if you replace a by nabla and aj by partial-d-by-dxj.
You can also write the determinant of a 3×3 matrix A as εijkA1iA2jA3k

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