Sarvadarshanasangraha 15.134

15.134

sā ca tapaḥsvādhyāyeśvarapraṇidhānātmikā kriyā yogasādhanatvād yoga iti śuddhasāropalakṣaṇāvṛttyāśrayaṇena nirūpyate yathāyur ghṛtam iti/

And that action consisting of austerity, recitation of sacred texts, and dedication to God is designated as "yoga" because it is an instrument of yoga, through recourse to the figurative function of pure superimposition (śuddhasāropā lakṣaṇā), just as "ghee is life."