An octave is the interval between one pitch and another pitch with half or double its frequency. For example, if one note has a frequency of 440 Hz, the note one octave above it is 880 Hz, and the note one octave below it is 220 Hz. These pitches sound very similar despite the large difference, and thus, when played simultaneously, they blend together, which can make it difficult for some people to recognize them as two distinct pitches.
In most Western music, the octave is divided into twelve evenly, or approximately evenly, spaced notes, and the most important scales are typically written using seven of these notes, plus a repetition of the first note one octave higher. An example is the C major scale, which is written as C D E F G A B C, the initial and final Cs are an octave apart. The other five notes are C sharp, D sharp, F sharp, G sharp and A sharp (which are the same as D flat, E flat, G flat, A flat and B flat). The term comes from the Latin word octavus, which means eighth.