Examples of this include prescription medicines such as insulin for diabetics.
An individual's income may vary, but their consumption of vital medicines remains constant.
[1] The second definition says that a good is neutral if the consumer is ambivalent towards its consumption.
That is, the consumption of that good neither increases nor decreases the consumer's utility.
For example, if a consumer likes texting, but is neutral about the data package on his phone contract, then increasing the data allowance does not alter his utility.