What tones n a seventh chord should not be omitted?

In a seventh chord, the essential tones that should not be omitted are the root, the third, and the seventh. The root establishes the chord's identity, the third defines its quality (major or minor), and the seventh adds depth and tension. Omitting any of these tones can weaken the chord's harmonic function, particularly in a dominant seventh chord, where the seventh is crucial for resolution. The fifth can often be omitted without significantly altering the chord's function.