The sheet below is depicting a "inderteminate chord": the piece is in F# minor and the chords have 2 notes each: F# and C#. THE CHORD DOESN'T INCLUDE THIRDS:
The modern definition is power chord. Used a lot by guitarists, often with distortion or overdrive.
'Indeterminate' can be used as it's neither major nor minor, having no 3rd.
It works well with guitar, as distortion highlights harmonics, and those of 1 and 5 sound fine together, but when a 3rd (either maj. or min.) is played as well, its harmonics clash with those others.
Otherwise known here as F♯5 - some argue that it's not even a chord, having only two notes, but a dyad. There are occasions in composing where the writer wants a harmony that doesn't fall into major or minor. This fits the bill.
The chord symbol for this, an open 5th on F♯, is 'F♯5'. Or maybe 'F♯(no 3rd)'. A symbol like 'F♯(no 5th)' could be used for an F♯-A♯ dyad.
'Indeterminate chord' is a rather vague description. If you intend to use it in an analysis I suggest you define your usage first.
Strictly speaking, the term "indeterminate" means that you cannot determine the quality of the chord with the information given. You might be tempted to use the surrounding context to determine that's it is most likely F#m, but even this claim is invalid.
It isn't a matter of not knowing if it's minor or major. Fundamentally, it is neither. It essentially has not been assigned a quality at all. This, in some sense, can almost be considered a third quality, a power chord.
Adding an A because "it's probably F#m" is no different from adding an E because "it's probably F#m7". You wouldn't be guessing information you don't know, you would be adding information that wasn't there before at all.