Success is achievement of something desired, planned, or attempted. However, just because an endeavor was unsuccessful does not mean it is without value. The failed Columbia mission and the Vietnam War are two such examples.
The goal of the Columbia space shuttle mission was to launch safely into space, perform scientific experiments, and to land safely on earth. This was shat the crew and the people at the Houston space center attempted. This mission was successful until reentry, when the shuttle disintegrated without warning. The goal was not achieved; seven astronauts tragically lost their lives. This failure does not mean this tragedy was without value. It forced NASA to reexamine its contingencies. NASA was compelled to look carefully at its organization structure, independent contractors, and engineering practices. Congress demanded accountability, and was forced to reexamine its budget practices concerning space exploration. Surely, this is a valuable thing to come from a horrible failure.
The goal of the Vietnam War was to prevent Communism from taking hold in that country. Many at the time believed that if Vietnam became Communistic, so too would the rest of the region. When we left, the country fell to the Communist party. However, advancements in military training came out of the conflict. We learned the value of "special forces" and developed new tactics to fight in environments where tanks were less efficient than air strikes. These lessons better prepared us for modern warfare. These important changes show the value that can come from failure.
It would be preferable if we did not have to pay such a high price to learn such lessons. The Columbia space tragedy and the Vietnam War both demonstrate how events perceived as failures can still have value.