What does a2 mean in an orchestral score?

A2 is an abbreviation for "a due", which is a musical term that means "two together". In an orchestral score, a2 indicates that two instruments should play the same part simultaneously. This is often done to add reinforcement or to create a fuller sound. For example, in a string section, the first and second violins might be marked a2 to play the same melody in unison.