Complexity deals with various forms of calculating the complexity of code. Cyclomatic complexity, n-path complexity, Big O time and space complexity.
Complexity takes many forms within computing. Most often it is used as an attempt to measure either how hard something is to understand, or how long (or large) something will run.
The understanding complexities that are used most often are that of cyclomatic complexity which looks at the number of independent paths through some code, and n-path complexity (more often seen in static analyzers) that looks at the combination of all the different paths throughout the code.
The more theoretical complexity is that of big O and is used with the asymptotic behavior of either the time code will take to run, or the amount of memory it will use.