By nature, a chord is three or more notes played together or in sequence. A two-note cord is more frequently called an interval.
In Western music, there are 11 notes arranged in a pattern of half steps and whole steps. Each note has an interval relationship to every other note in the scale. Every chord is made up of one or more intervals.
The distance between C and D is a whole step. On the piano keyboard, you can identify a whole step by moving two chromatic spaces up or down. One chromatic space is a half step. A set of notes with seven chromatic steps between them is identified as a 5th.
A two-note chord can be as large or as small as you want. The smallest interval unit is a half step, also known as the chromatic step. You can expand the interval to be as large as your keyboard or instrument can handle.
In Western music, an octave is the interval that occurs when you play two of the same note names spaced one major scale apart. An octave interval gives a powerful boost to the root when played as a two-note chord.