The results of an Analogue to Digital Converter's (ADC) conversion are represented as a digital number with varying resolution depending on the bit-length of the converter and settings used. You can often have 8, 10, 11, 12, 16, and even 24 bit ADC peripherals in microcontrollers or dedicated external ICs.
The results are obtained usually by a sample-and-hold input setup, which using a known conversion time, the resulting voltage built up on the sampling capacitor is the representation of the external signal input.
The results are digitized as a proportion of a reference voltage given to the ADC. If the input signal is 1V, with a 3V reference, then the result (no matter what resolution you have, be it 8 bit or 24 bit) will be 1/3. The better resolution ADC obviously gives you much better ability to distinguish between 1.001V and 1.002V, or similar difference in input signals. If you are doing small-signal detection, like the PPM of a certain gas in a gas sensor output signal, then higher resolution and often a lower reference voltage is commonly used.
For rough voltage signals, like the state of charge of a lithium ion battery pack for a hobby project, a simple 8-bit ADC is enough to work out if you are getting low, or need to turn off the charge, etc.
The conversion result's units are merely a proportion of the reference given to the ADC. If you have an imprecise or noisy reference even if your input signal is steady, you can have errors. Either way, the main point here is there is not really an easy unit to refer to ADC conversion results.
According to the University Of Chicago they refer to the results of an ADC conversion as "raw values" and are: "The digital number output by the ADC; the units in which raw values are measured are called ADU (analog-to-digital units) or DN (data numbers). "
I agree with the ADU unit of measure, it seems nice and keeps in the context that it's a ADC conversion output, so anyone reading should immediately be aware of the proportional issue. If not, then the reader should be informed prior to mentioning the units of their significance.
They can be converted to percentages merely by dividing the raw value given by the ADC by the maximum possible value given the resolution of the converter. For example if your 8-bit ADC returns 100, you can say that it is (100/255) * 100 = 39.21% and can be seen as more of an "intensity" or "signal strength" in that case; however it really depends on the scenario if this is useful to think of this way.
Often the raw values are immediately converted to something that makes sense, such as a battery voltage (after a voltage divider factor has been applied, for example) or a dB intensity for sound/noise. The reference to ADC conversion results is not very common, and as you've seen it's usually because without prior reference or meaning it's hard for readers to understand it's significance. Another example may be the output of a Gyroscope sensor's X axis, which has an analogue output with a particular gain formula given by the manufacturer, and the analogue result should be processed immediately to give you a more useful unit of 'degrees per second', which people (and any further use in the program) can usually make better sense of.