Correlation is defined by dict.org as:
- a statistical relation between two or more variables such that systematic changes in the value of one variable are accompanied by systematic changes in the other 
In other words, if you are studying two variables and both change at the same time (consistently, veritably, and repeatedly), then you still cannot conclude one causes the other. Instead all you can say is that there is a link between the two, but the directionality of that link still needs to be established by negative testing. The design of the system must also be reexamined to assure that the design is not causing a systematic connection between the two variables, by implicit means.