Premature optimization is making an optimization for performance at the cost of some other positive attribute of your code (e.g. readability) before you know that it is necessary to make this tradeoff.
Usually premature optimizations are made during the development process without using any profiling tools to find bottlenecks in the code. In many cases the optimization will make the code harder to maintain and sometimes also increases the development time, and therefore the cost of the software. Worse... some premature optimizations turn out not to be make the code any faster at all and in some cases can even make the code slower than it was before.
