A wage/regular income paid by the government.
We've paid into for 40+ years, unlike benefits which can be paid to someone who just sits on their ass and have never paid taxes/NI in their entire lives.
I get that's how it's sold to everyone. And no-one is saying that it shouldn't work like that. But the reality is that when you say you "pay in," that suggests some sort of investment that it saved for your retirement. In reality, National Insurance is just another tax that is immediately spent, and you are then promised a cut of future taxpayer's tax payments to pay for your retirement. Effectively, there is no relation between what you paid in and what you get out. Your contributions paid for your parents'/grandparents' pensions, and my contributions will pay for yours.
And that's fine, as long as the unofficial contract between the generations holds up. It's not fine when the state pension (as modest as it is) is ring fenced while working people's wages are stagnating, and nurses, train workers, teachers, etc, are told they can't get a pay rise to match inflation. If for no other reason than the practical reality, which is that if working people aren't paying enough taxes, there'll be no money to pay for your pension anyway.
Obviously, I'm very aware that it risks turning into this battle where generations turn against each other, rather than turning against the people who got us into this mess. But I think it's important that pensioners face up to the economic reality of how their pension is paid, and if you keep voting for (not all, but in higher numbers than any other age group) isolationist, tax-avoiding politicians that fuck up the economy, there won't be enough money to 'pay out' your pension no matter how much you 'paid in.'
The other option, of course, is to switch to an entirely individual model, where your National Insurance contributions go directly into a government-operated savings account, are invested on your behalf and can't be touched by anyone until your retire. Then you really will get out what you put in. But it'll take 50 years to transition to something like that, and in the meantime, we've still got to pay for pensioners somehow.