The critical role played by University of California lecturers was affirmed Wednesday in a tentative labor agreement that averted a massive strike and widespread class cancellations — instead sparking jubilation over what the union called the “best contract” in its history.

The 6,500 lecturers teach one-third of undergraduate courses and are praised by campus members as indispensable partners who provide instruction, do research and guide students, often spending extra time to help with assignments or write letters of recommendation. But they are non-tenured and had been working without a contract for 20 months as they pressed for better job security, pay increases and family leave benefits.