Index fund
A fund that copies a published market index, holding the same securities in close to the same weights, so its return tracks the index instead of trying to beat it.
An index fund is built to copy a benchmark rather than outguess it. The manager holds what the index holds, in the weights the index uses, or a sample close enough to behave the same way, and trades mainly when the index itself changes. There is no research team to pay for and little trading to pay for, which is why index funds usually carry a much lower expense ratio than funds that pick their holdings.
Two funds tracking the same index own the same things, so the ways they can differ are narrow: cost, how closely they track, and how they are wrapped. The same index is commonly sold both as a mutual fund and as an ETF, and choosing between those wrappers is a question about trading and tax handling rather than about what you own.
The mistake is hearing index fund as a synonym for safe or diversified. It says only that the holdings are chosen by a published rule instead of by someone's judgement. An index of one industry, one country or twenty large companies is exactly as concentrated as it sounds. The index name is the part worth reading, not the fund's.