It is to say a dominant seventh without the degree II of root tonality.
For example, in Clara Schumann's lied Was weinst du, Blümlein, we have B-A-B-D#. In this bar we are in E major ; this chord seems like a dominant seventh without F#. How can I make its chord notation (with roman numerals) ?
Full score (p. 2, system 2)
A chord will need its root present - otherwise we don't know what to call it.
It needs its third, as without that, we don't know if it's a major or minor chord.
If it's a seventh chord, it obviously needs that 7th note - of some sort - here m7, making the chord dominant seventh, as you anticipate.
There is often the omission of the 5th (P5) from a chord, as that pitch is present in the harmonics of the root, and is the first, generally, to go, in a complex chord. Obviously, if it's ♯5 or ♭5, it'll need to be there, but in a 'standard' chord, it adds not that much to the sound (unless it's a 'power chord', but that's a different kettle of fish!
So B, D♯ and A notes are sufficient to make the B7 chord, in just about any genre of music.
EDIT: apparently the piece is in key A, so the B7 chord would more likely be in RN seen as V7/V. Thanks to phoog.
