Transient Stability Assessment (TSA) is the process in which the stability of a system is characterized qualitatively or quantitatively. The TSA algorithm presented in this paper is derived from the well-established Single Machine Equivalent (SIME) method and can thus be categorized as a hybrid direct-temporal method. The novelty of the proposed algorithm is that it derives a Transient Stability Index (TSI) with a single Time-Domain (TD) simulation for both stable and unstable cases. The resulting TSI is uniform in units and linear around the instability point. Results are reported for two sample power systems of 9 and 36 buses. The proposed algorithm has also been successfully employed to speed-up a Critical Clearing Time (CCT) determination algorithm.