A special notation for writing vector equations using the scalar components of vectors rather than the vectors themselves. This notation has found widespread use in the study of atmospheric turbulence, where the many vector and tensor components of a governing equation can be concisely represented by a much smaller number of terms. The key to this notation is the use of indices (subscripts) i, j, and k, which can each take on the values of 1, 2, or 3 to represent the three Cartesian directions (x, y, z). See also alternating unit tensor.