Quote vs. estimate vs. proposal
These three terms are often used interchangeably but they carry different weight. An estimate is a best-guess number — the client understands that the final price may differ if the job proves larger or smaller than expected. A quote is a firm price offer: if the client accepts, that is the price, and you are on the hook for any overrun unless the scope changes.
A proposal is broader still — it includes the price but also lays out the approach, the team, the timeline and often the reasoning behind the recommendation. Proposals are appropriate for larger engagements where the client is deciding between vendors on more than price alone.
Match the format to the situation. A ten-minute plumbing visit needs an estimate; a bathroom renovation needs a quote; a six-month agency retainer needs a full proposal. Sending a proposal for a small job wastes everyone's time; sending a bare estimate for a large one signals that you have not thought the work through.