Basket, in the financial context, refers to a number of securities bundled together that can be bought or sold together for investment purposes or returns.
The term, basket, is usually used for an exchange traded fund (ETF). An ETF contains a securities basket which is traded on an exchange. An ETF basket has a makeup similar to the benchmark index the ETF tracks but of course, in multiples of the same ratio of securities as the benchmark.
A Nifty 50 ETF will be a basket in which all the 50 stocks of the Nifty 50 index will be present.
Basket order, another commonly-used term, means a transaction order on a group of securities at one go.