A balanced fund is a mutual fund that includes ownership of stock, bonds and sometimes money market funds. Unlike a target-date fund, the percent of funds allocated to each category is mostly fixed, e.g. 70% stock and 30% bonds.
An example of a balanced fund is Vanguard’s balanced index fund VBAIX, which maintains 60% stock and 40% bonds. The stock and bond components themselves are widely diversified, hence a fund like this can act as the only fund in a portfolio.
Investopedia's article on balanced funds