A word of advice is normally advice given about an imminent situation. The advice could be cautionary, for example, advice given by a father to his son to avoid a difficult situation, or it could be positive advice such as advising something for a positive action.
A piece of advice is reference to a single piece of advice. To quantify the word advice you can also say two pieces of advice or three pieces of advice.
Please bear in mind that the word advice is an uncountable noun and you must never say 'an advice', which is wrong.
The verb form of advice is advise. When you give advice to people you are advising them.
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