This work presents a novel concept and formulation of the "Phasor Area Criterion" (PAC), which assesses the qualitative and quantitative aspects of a power system's transient stability using time synchronized measurements. Unlike most existing techniques, the PAC method is computationally inexpensive and free from state estimation of rotor angles. It utilizes direct synchrophasor measurements from a phasor data concentrator to assess the system's post disturbance transient stability status and identify the out-of-step generators. The PAC extracts the area of the convex hull from the trajectory of positive sequence voltage phasors within the phasor domain; therefore, the criterion outputs a monotonically increasing parameter that can assess the transient stability of the system. Thus, it is robust to false triggers due to multiple crossovers with the thresholds. The PAC algorithm is validated on Kundur's two area system, IEEE 39 bus system with different operating conditions, and also for the <inline-formula><tex-math notation="LaTeX">8^{th}</tex-math></inline-formula> January 2021 system split event for the European grid. Compared to the transient stability index, PAC delivers high situational awareness with accurate and early alerts of transient stability assessment.