This paper considers thought experiment as a special scientific tool that mediates between theory and experiment by mental simulation. To clarify the meaning of thought experiment, as required in teaching science, we followed the relevant episodes throughout the history of science paying attention to the epistemological status of the performed activity. A definition of "thought experiment" is suggested and its meaning is analyzed using two-dimensional conceptual variation. This method allows one to represent thought experiment in comparison with the congenerous conceptual constructs also defined. A similar approach is used to classify the uses of thought experiments, mainly for the purpose of science curriculum.