Triad - A group of three notes
Note: a triad is also known as a chord.
To remember this, recall the following image:
A triangle – add (triad) them up and there are three corners.
In more detail, a triad is a group of three notes (a three-note chord) that look like they have been stacked vertically on sheet music one line away from each other.
The above are all triads.
To get more technical, these are a three-note chord whose pitch classes can be arranged as thirds.