What is Analysis Of Alternatives?

Legal Definition
The Analysis of Alternatives (AoA) in the United States is a requirement of military acquisition policy, as controlled by the Office of Management and Budget (OMB) and the United States Department of Defense (DoD). It ensures that at least three feasible alternatives are analyzed prior to making costly investment decisions. The AoA establishes and benchmarks metrics for Cost, Schedule, Performance (CSP) and Risk (CSPR) depending on military "needs" derived from the Joint Capabilities Integration Development System process. It moves away from employing a single acquisition source to the exploration of multiple alternatives so agencies have a basis for funding the best possible projects in a rational, defensible manner considering risk and uncertainty.
-- Wikipedia
Legal Definition
When choices are evaluated to get to the same goal. AKA alternative analysis.