Dominant Seventh (7)

Root + major third + minor seventh

Prove it — just do the practice exercise.
C7 Shell Voicing
C (R)E (3)Bb (♭7)
A Dominant 7 shell voicing consists of 3 notes:

Root
Major 3rd — 4 semitones above the root
Minor 7th — 10 semitones above the root

Example: C7 = C + E + Bb

The difference from Maj7: the seventh is one semitone lower.