what's an unrealized gain/loss in a 401k

For a 401K -- or any other tax-deferred account -- gains are only realized when you sell and move the money to something else. Your statement showing unrealized gains is probably just an artifact of the fund company's standard template sent to investors in both taxable and tax-deferred accounts. It really has little relevance in your case.

If they're using the term the same way as it would be used in a taxable account, it means the price appreciation in the fund since you purchased it.

Never. Well, not until you sell.

It never gets "posted" absent you selling them.

It doesn't "post". If you buy $10,000 of Fund XYZ in your 401(k), and a year later your XYZ holding is worth $25,000, you have a $15,000 unrealized gain. But it could easily shrink/disappear if Fund XYZ drops in price. The only way to "post" it is to sell XYZ.

so since I *never* do move or sell anything but have stayed in the mix of funds since I joined the 401k plan, it means literally nothing? 25k a year is a big deal and I just now noticed it on the last online statement so am wondering at what point it's actually booked?

I don't actually get a statement, mailed, it's all online only and I access the account only online somehow the 401k has also continued to grow over the decades....have had same plan, same funds since 1979

At no time does the fund book the gain/loss until you sell the fund, or when the gains are distributed to you (most likely annually).

In a taxable account, it allows the fund holder to do some tax planning in anticipation of a fund distribution.

Not really, since it's in a 401K account, it shouldn't have any tax consequences.

All it means is that at the moment the statement was prepared, your investments in the account were worth $25,000 more than you paid for them. That's it. The market could crash tomorrow and they could suddenly be worth less than you paid for them. Or they could keep going up. But the key part is that $25,000 gain is not and will never be guaranteed, booked, or posted in any way (unless you sell and thereby convert the paper gain to cash).

Or until they distribute it to you. But either way it's not a taxable event since it's in the 401K.

Isn't that wonderful?

I believe that if you have also been reinvesting all dividends and capital gains in the fund that those become realized gains, therefore using your numbers; if you buy a fund for $10,000 and reinvest $2000 of dividends and long/short capital gains, and at the end of the year the fund is worth $25,000, you end up with $13,000 unrealized gain i.e., had you not reinvested but had taken the dividends and cap gains in cash instead, you would end up with a fund worth ~$13,000 and have $2000 cash (a realized gain).

But as someone said: in a tax deferred account, like a 401K, it doesn't really matter what you call it, because it doesn't matter one way or the other until you sell the fund.

That's certainly correct and true.

However, many brokers/custodians I have come across will, for tax-deferred accounts, list economic gains as "gains" or "unrealized gains" -- ignoring reinvested dividends. In a way that's correct -- if you put in $10,000 and it's worth $35,000, you really do have a $25,000 economic gain. It's calling it "unrealized gain" which is a specific term that really only has full meaning in the world of taxable accounts that's the problem.

Note that in a 401K the buying and selling of funds and reinvestment or not of dividends and interest has no tax consequences. The tax consequences occur when money is withdrawn.

However, one nuance that does matter is net unrealized appreciation in company stock in your 401K which does receive special tax treatment when rolled over or withdrawn. See for example

many articles that can be found on this subject. Note thatoften your company 401K policy may prohibit repurchasing company stockif you once sell that stock. One statistic you might ask about your 401K if you hold company stock is what is the NUA amount on that stock as distinct from "unrealized appreciation" in one fund or another.