The multiple correlation coefficient denotes the correlation between one variable and multiple other variables. It is represented as R1.234...k, where 1 is the variable being correlated and 2, 3, 4, etc. are the other variables. As an example, R1.23 would represent correlating variable 1 with variables 2 and 3 simultaneously by creating a linear combination of 2 and 3. The document then discusses using multiple correlation to correlate academic achievement with a linear combination of anxiety and intelligence.