The phrasal verb "stand by" is unique and has several different uses. It can mean to await something, to remain inactive in some situation, or to be loyal or faithful to someone or something.

Both of the previous answers are correct, although the first is probably a bit more common. It just depends on whether 'stand by' is being used as a verb (command) or as a noun. For the noun form 'stand in' could be used as a synonym.

Standing is well known word ans assumes that you know it mean, while stand by can be use for spare for any machine, like if any spare motor can use when any running motor had stopped working and when we use other motor in this case it is known as stand by .