(Eng., Ger., Fr., It., Sp., la). 1. The sixth note of the C-major scale. The frequency for A above middle C is 440 cycles (vibrations) per second. 2. In some methods of musical analysis, uppercase A stands for the A-major triad, lowercase a for the a-minor triad. 3. The note used for tuning an orchestra, usually given out by the oboe because the oboe is the least affected by humidity or other weather conditions of all pitched instruments.