A target date fund is one investment-fund, picked based on the year you’re going to retire.

E.g. if you’re retiring in 2065, go and look for brokers that offer 2056 funds (Vanguard 2065, Fidelity 2065, etc).

These funds are automatically diversified among different stocks and bonds. As you get older, the fund gets more conservative.

References

• key-metrics-for-personal-finance