What is the full meaning of POS?

POS means point of sale and refers to a point in time and place where a transaction between a merchant and a customer is completed.

At the POS, a merchant calculates the amount owed by the customer. The merchant may prepare an invoice or use a point of sale software to present the customer with payment options.  

After payment the merchant may print a receipt or send one electronically to the customer.

A POS is sometimes referred to as a point of purchase (POP)