What are identical molecules?

I don't know what identical molecules are but these are by thoughts.

Since from the word identical the molecules should have the same number and kind of atoms attached in the same way. Since in stereo-chemistry the isomerism is due to spacial orientation all stereo-chemical isomers are identical.

Is my thinking correct? What factors determine whether two molecules are identical or not?

Of the two types of stereoisomers, diastereomers (non-mirror image stereoisomers) are clearly nonidentical. Consider the isomers of tartaric acid. Meso-tartaric acid and (+)-tartaric acid have different melting points (165 $^\circ$C and 171 $^\circ$C, respectively). They also have different acidities.