Payment Date
  
See: Dividend. See: Declared Date.
The Pay Date, or Payment Date, is the day a dividend actually pays.
Like...you own 1,000 shares of Duke Energy. They pay fifty cents a share in dividends 4 times a year. You owned your shares past April 15, which was that quarter's "minimum date" that you needed to own those shares to be eligible for the dividend. It was declared. You were owed the dough. Now you just had to wait 3 weeks until that dough is actually paid into your account.
This whole notion of so many delays in payment on dividends came from the very low-tech and slow world of a century ago. Back then, for many, the only gains investors made on investments comprised dividends. Stocks often really didn't go up. So the dividends mattered, and when you owned the stock mattered. Things weren't registered and tracked electronically, so some poor scrivener had to receive proper legal notice of you having bought your shares ahead of that key date so that you were eligible for the divvy. Then there was a paper check he wrote. And an envelope he put it in. And then postal workers, oft angry, who had to deliver it to you n weeks later.
With electronic deposit and automatic connections between most brokers and most public companies, the process is today lightning fast and relatively easy for all. But the relics of time delays stil lexist in the process, so we...deal.