Can someone give me a real dumbed-down definition of what an equity index fund is?

Answer 1

"Equity" means stocks
"Index" means one of the names you always hear on the radio, like "The Dow Jones is up today" or "The NASDAQ moved higher in trading" - Dow Jones and NASDAQ are indexes. As the Dow Jones goes, so goes the rest of the market. An index keeps track of some stocks, and you presume all the rest are going in the same direction.
A "fund" is a group of something. You buy the fund and so you have a whole bunch of investments.
Therefore, an equity index fund is a group of stocks that is commonly tracked, and you buy the whole bunch of them in one purchase.

Answer 2

There are various indices that measure the performance of the US Stock market. Like the Dow Jones Industrial Average and the Standard & Poors 500 Stock Portfolio.

An equity Index fund buys the stocks (or mimics) the performance of that stock index.

So, let's say you buy into a Standard & Poors 500 stock index fund. You should get the same performance in terms of dividends and yields as if you had invested in all 500 stocks.

Answer 3

It's a fund that holds a basket of stocks that comprise a certain index (S&P 500, Russell 1000, etc.) and therefore tracks the performance of that index.