A proffer is an offer made prior to any formal negotiations.
In a trial, to proffer (sometimes profer) is to offer evidence in support of an argument, or elements of an affirmative defense or offense. A party with the burden of proof must proffer sufficient evidence to carry that burden. For example, in support of a particular argument, a party may proffer documentary evidence or witnesses.
Where a party is denied the right to introduce evidence because that evidence would be inflammatory, hearsay, or would lack sufficient authentication, that party must make a proffer of what the evidence would have shown in order to preserve the issue for appeal.
As in business, a proffer can be a sign of "good faith" a first offer or proposal, to show a willingness to "barter".
Etymology
The word proffer is derived from Anglo-French "por-", forth, and offrir, to offer.