An ELT Glossary : Contrastive Stress

Definition : Stronger emphasis (increase in loudness, length, higher pitch) on one syllable in order to contrast it with another and therefore to "correct" an assumption or implication in another part of the discourse. This often means the stress shifts to a syllable where it would not normally occur.

Example: A : Did you ask Mr Evans? / B : No, I asked 'Mrs Evans - with contrastive stress on /mɪs/ to contrast Mrs with Mr in the previous utterance.

Example :  A: I thought you said it was necessary? / B: No, I said it was 'unnecessary. with contrastive stress on the first syllable "un" (whereas normally the word would be pronounced with the stress on the second syllable "nec"

Extra information : Understanding and producing contrastive stress can be difficult for learners who don't use it in their own language - eg Italians