A missing value in a dataset is when a field that is expected to have a value has none, for example if the hourly reading from a sensor failed. This is related to an optional value, which is when a field can quite reasonably have no meaningful value, for example, the heart rate in a medical record if no measurement was taken. Note that in both cases the absence of the value may itself carry information (e.g. that the sensor has a problem) and is different from a zero value.