A Pricer calculates the Implementation Shortfall (Market Impact) of a transaction for a specific price.

A Pricer is a piece of software which calculates the transaction costs. 

A Pricer is answering electronically answering an RFQ (Request for Quote) automatically.

A Pricer can also be called Pricing Engine.

A Pricer is a piece of software which calculates fair prices of financial instruments mostly based on live market data.

A Pricer is a piece of software in electronic trading which automatically calculated ideal prices of financial instruments.

A Pricer is a piece of software which uses algorithms (simple and complex) to determine a fair price for a financial instrument.