Buy is a word that can be used in a variety of circumstances. In the investing world, it includes the following: 

1. Obtaining ownership of a good or service in exchange for payment, either in cash, credit, or the exchange of goods or services. When purchasing a financial instrument through a broker, ownership is taken from the seller, and typically the ownership is not physical.

2. A buy recommendation from a broker or third party for a financial instrument, which they believe will increase in price due to economic events, and is therefore profitable to purchase.

3. A buy order, where a trader tells a broker to buy a financial instrument in the form of assets or securities. A buy order can be known as “going long.” 

References:

https://www.ig.com/en/glossary-trading-terms/buy-definition

https://www.investopedia.com/terms/b/buy.asp

https://www.collinsdictionary.com/dictionary/english/buy