VISA - charta visa, in Latin (means, paper that has been seen)
It is a document issued by a country to a person to formally ask the traveling country for permission, with a specific reason for a stipulated amount of time. All the countries have their own VISA laws and it is up to the country to allow a traveler to allow him/her or not. A visa does not guarantee entry, a person may also may also be rejected after getting permitted.
A VISA can be of many types like study, business or work. Like, if you want to pursue study in US, you need to get a study VISA of US.
VISA - Visa International Service Association
Visa International Service Association is a multinational corporation based in San Francisco, California, USA. It issues and manages products including credit cards and debit cards which can be used internationally for retail payment, online transaction, ATM, merchants, managing payments among financial institutions and more.