An instrument is indiscrete when it doesn't have easily definable separate parts. This can be formed in two primary ways, when the instrument has parts that blend into one another, or when it has many individual parts that combine to make a whole in a way that the relationship is difficult to define specifically.
Parts blending together
This is like a hybrid instrument where a harp that blends into a guitar or a ukulele that is also a flute.
An excellent example of this is the Skatchboxes made by Tom Nunn which are large wooden boxes covered in various plastic combs, rods, and corrugated materials. Is each individual comb an instrument or is the mass of these combs and rods combined the instrument? This relationship where there is uncertainty of where the boundaries of instrument are is the key feature of being indiscrete.
Many parts making a whole
Another form of being indiscrete is when a swarm of instruments have been made that play together at the same time, this is a common strategy in sound sculpture. An example of this is the installations made by Zimoun. Many of his installations have exact copies of the same sound making object, such as a box with a small motor moving a wire that taps against the edges of the box. This exact component will be made hundreds of times over, which each playing at the same time in the same space. Is the instrument each component or the mass of these playing together? This distinction of the mass versus the individual components is a key feature of the instrument being indiscrete.
Many indiscrete instruments also have fungible parts, and those parts may be fungible in various ways. If a single box is removed or added from a Zimoun sculpture is that the same instrument, and to what degree is it changed by that aspect. Often these sound sculptures are made of many fungible parts and the overall work is ambiguous in where the boundaries are important.
This concept needs some further clarification, this ideas are in no way fully fleshed out, so if this page feels.. off.. thats because it is.