Pitch
Pitch is how high or low a sound is. Two pitches sound the same if they're the same frequency; different if they aren't.
Note
A note has a name (A, B, C, …) and a pitch. The same note name in different octaves sounds related — that's the basis of an "octave".
Interval
An interval is the distance between two notes. A 5th sounds bright and stable; an octave is two notes that sound "the same" but one is higher.
Chord
A chord is several notes played at the same time. The "quality" of a chord (major, minor…) describes its mood.
Scale
A scale is a sequence of notes within an octave that establishes a key. The major scale (Do Re Mi Fa Sol La Ti Do) is the most common in Western music.
Key
A key is a "home" pitch (the tonic) plus the scale built on it. C major has C as its home; songs in C major feel resolved when they land on C.
Solfège
A movable-do system that names notes by their function in a key, not by letter. C in C major is "Do"; C in F major is "Sol". Useful because it travels with the key.
Contour
The shape of a melody — whether it ascends, descends, or both. Easier to hear than specific intervals, and a good warm-up.