What is the meaning of the word "Sutra" appearing in the Abhivadanam?

Sutra refers to the Grihya/ Srauta Sutra according to which one has to perform the various Nithya and Naimittika Karmas as per Vedic tradition. For example, for Krishna Yajur Veda , Apastamba Sutra is the prescribed Sutra . One who is born in this lineage has to perform the Vedic rituals in Krishna Yajur Veda as ordained by Apastamba Rishi in the Apastamba Grihya/ Srauta Sutras.