A prompt is all the text an AI receives to do its job: your request, the instructions, the context and any examples you add. It is the only thing the AI knows about what you want right then, so much of the answer depends on how clear it is. In a chat, the history usually goes along with it.
A question from the app. Pick an answer.
What is a prompt?
It is the input you give it: instructions, questions or examples. A clear prompt can help.
The model processes input. The prompt is that input, not the model's name.
Making things up is called a hallucination. The prompt is your request.
A good prompt usually has four parts: the task (what you want), the context (for whom, what for, with what data), the format (list, table, three lines) and, if needed, an example of what the result should look like. You do not always need all four, but when the answer is not useful, one of them is almost always missing.
Some parts are not written by you. Many apps add their own instructions before your message, and in a chat the earlier conversation is sent again too. All of that takes up tokens and goes into the context window: if the chat gets very long, the beginning can be left out.
Vague: "Give me birthday ideas." Clear: "Give me 5 ideas for my daughter's 8th birthday, at home, with 12 kids and a budget of 100 USD (an example number, use yours). No screens. For each idea, a list of what to buy." The first gets you a generic list; the second, something you can use on Saturday.
If you are not sure what to ask for, a useful trick: end with "before you answer, ask me any questions you need." The AI asks what is missing and you fill in the context without having to guess it.
Many rules depend on where you live. Ask "how is overtime calculated?" with nothing else and the answer may not fit you: it changes between countries, and in the United States it can also change by state. The same goes for taxes, paperwork, currency and units. Adding "I live in Texas" or "in Canadian dollars" changes the answer more than any clever wording.
"There are magic words." There are no secret phrases: what works is saying what you want, for whom and in what format. "I ask for five things in one message." It often does two well and forgets the rest; go one at a time. "It remembers yesterday's chat." It depends on the app; often each chat starts from zero. "A clear prompt stops it from being wrong." It helps, but it can still make things up: check what matters. And never paste passwords or other people's private data.