These three documents appear at different stages of a client relationship, and mixing them up can create confusion or even pricing disputes.
Quote
A quote is a fixed price offer for a defined scope of work, given before the project starts. Once accepted, it's generally treated as a firm commitment on price.
Estimate
An estimate is an approximate cost, used when the scope isn't fully defined yet. It signals 'this is roughly what it will cost' rather than a locked-in number, and the final invoice may vary from it.
Invoice
An invoice is the actual request for payment, issued once work is delivered (or at agreed milestones). Unlike a quote or estimate, it represents money currently owed.
Why the distinction matters
If you send an 'estimate' but a client treats it as a fixed quote, disputes happen when the final invoice is higher. Label your documents clearly and explain upfront whether a number is fixed or approximate.
A simple rule
Quotes and estimates come before work begins and are not requests for payment. Invoices come after work is delivered and always are.