1. Correlation between corresponding members of two or more series: if q₁, ..., q_n and r₁, ..., r_n are two series, correlation between q_i and r_i , or between q_i and r_i+j (for fixed j ), is a cross correlation.

2. Correlation between or expectation of the inner product of two series of random variables, where the difference in indices between the corresponding values of the two series is fixed.