Is TD Ameritrade instant reinvestment?

When you sell stock, the cash does not come back into your account until the 3 business day settlement period. But money from the sale should immediately be available to buy something else even though it has not settled yet because you will have the money by the 3 day settlement of the purchase. But as the anonymous person said, you have to be aware of the "free ride rule", so if you immediately buy something with unsettled money, you cannot sell it until the first sale settles. In other words you cannot buy "and" sell something before you had actual (settled) cash to buy it.

In North America, all security trades are posted to an account as of the settlement day of the trade.

That is, all cash purchases are posted and must be paid in full by the settlement day and all sales are posted to the account and all funs are available on the settlement day.

There is no such thing as "a three day holding period" since funds, by law, are due/available on the settlement day.

Proceeds are not considered "reinvested" they are simply posted to the account.

It doesn't matter what firm you use, all firms must follow the same rules.

The proceeds are immediately credited to your account. You can immediately use those funds for another purchase, but you cannot SELL that purchase until the prior sale and purchase clears (which takes three days).

If you buy a stock on Monday you don't actually pay for it until Thursday. If you buy a stock with unsettled funds and then sell the stock that you still haven't paid for, you're Free Riding.

Remember the mantra "T+3" (trade plus three days).

Look up the Free Riding rule.