Huacatay (pronounced "wah-ka-tay") is an herb with strong aromatic flavor that is used in many traditional Andean dishes. The delicious but somewhat mysterious green sauce that is often served with pollo a la brasa - Peruvian roast chicken - is typically made with huacatay (and aji amarillo chile peppers). Hucatay is also known as Peruvian black mint, and is native to South America. The plant is in the marigold family (genus Tagetes).
It's not easy to find fresh huacatay in North America (though it's apparently fairly easy to grow), but you can find jarred huacatay paste in many Latin food markets, right alongside the aji amarillo, rocoto, and aji panca pastes.