A call is an action that can refer to two distinct but related financial instruments. The first is a call option and the second is a call auction, each with a distinct purpose and set of outcomes.

A call option grants the buyer of the instrument the right, but not the obligation, to purchase a specific underlying instrument at a predetermined strike price within a predetermined period of time. This instrument is commonly used by speculators who are hoping to benefit from upward price movements, as well as by investors who wish to hedge their risks or write covered calls.

A call auction is similar to an auction, but it is conducted periodically and follows a set schedule. During a call auction, all interested participants can submit bids and offers for a given security, and the price of the security is then determined by the value at which the largest order on the book is filled.

Call auctions are most often used in illiquid markets where it can be difficult for traders to accurately determine the price for a given security due to lack of reliable and consistent order flow. By setting up a call auction, traders can ensure that the security trades at a fair price determined by market conditions as opposed to being set by a single participant.

In summary, a call is an action related to either a call option which is used to speculate on price movements, or a call auction which is used to set the price of a security in illiquid markets. Calls are an important part of modern financial markets as they ensure an efficient and fair way to price assets, and provide market participants with access to a variety of opportunities for gains.