What are controlled conditions?

An 'experiment' was defined as data collection under controlled conditions- but what does 'controlled conditions' mean?

It means that variables that can affect the data collected from an experiment are monitored and held constant. Control is essential for two reasons: Natural phenomena involve many interrelated variables, and if some of these variables are not held fixed over the course of the experiment, patterns in the data may be difficult or impossible to discover. To ensure that the data is reliable, an experiment must be repeatable and the data must be reproducible. Controlling conditions makes it possible for others to duplicate the experiment.