Spirit's NASA mission designation in the Mars Exploration Rover mission was MER-A.
NASA paired the two rovers by letter: Spirit was MER-A, while its twin Opportunity was MER-B. Spirit launched first, on June 10, 2003, and landed in Mars's Gusev Crater on January 4, 2004.
The designation is easy to confuse with MRO, the Mars Reconnaissance Orbiter, and MSL, the Mars Science Laboratory mission that delivered Curiosity. MER-A also appears alongside the alternate designation MER-2, but MER-A is the standard mission pairing used for Spirit in the rover program.
Spirit was designed for a primary mission of about 90 Martian sols. Instead, it operated for more than six years, studying volcanic rocks, ancient water-related evidence, and the unusual terrain around Gusev Crater before losing communication in 2010.