What is Legal Tender?

Legal Definition
Legal tender is a medium of payment recognized by a legal system to be valid for meeting a financial obligation. Paper currency and coins are common forms of legal tender in many countries. Legal tender is variously defined in different jurisdictions. Formally, it is anything which when offered in payment extinguishes the debt. Thus, personal cheques, credit cards, and similar non-cash methods of payment are not usually legal tender. The law does not relieve the debt obligation until payment is tendered. Coins and banknotes are usually defined as legal tender. Some jurisdictions may forbid or restrict payment made other than by legal tender. For example, such a law might outlaw the use of foreign coins and bank notes or require a license to perform financial transactions in a foreign currency.

Generally, designation of a particular form of money as legal tender means “that the designated money is valid payment for all debts unless there is a specific agreement to the contrary.” In some jurisdictions legal tender can be refused as payment if no debt exists prior to the time of payment (where the obligation to pay may arise at the same time as the offer of payment). For example, vending machines and transport staff do not have to accept the largest denomination of banknote. Shopkeepers may reject large banknotes: this is covered by the legal concept known as invitation to treat. Under the law, United States money as identified above is a valid and legal offer of payment for antecedent debts when tendered to a creditor. By contrast, Federal statutes do not require that someone who is not a pre-existing creditor must accept currency or coins as payment for goods or services. Private businesses may formulate their own policies on whether to accept cash unless state law requires otherwise.

The right, in many jurisdictions, of a trader to refuse to do business with any person, means a purchaser may not insist on making a purchase and so declaring a legal tender in law, as anything other than an offered payment for debts already incurred would not be effective.
-- Wikipedia
Legal Definition
That kind of coin, money, or circulating medium which the law compels a creditor to accept in payment of his debt, when tendered by the debtor in the right amount.
-- Black's Law Dictionary
Legal Definition
A tender in legal tender notes or United States coin. See 17 Ind. App. 444, 46 N. E. 151.
-- Ballentine's Law Dictionary