What is the Full Form of API ? What is API Key ?

API Key

The Application programming interface key is a unique identifier that uses different programming or scripting languages to authenticates the request associated with the projects.
For example youtube API 3 , Google Map Api etc.
Usage of API Key for HTTPs, API keys used in http in different way as shown below:
As a query string:
POST /codenaive/v3/search?key=[X_API_KEY] HTTP/1.1

As a request header:
GET /codenaive?example HTTP/1.1
Authorization: Bearer [YOUR_ACCESS_TOKEN]
Accept: application/json

X-API-KEY=
As a cookie:
GET https://xyz.codenaiveapis.com/codenaive/v3/search?key=[X_API_KEY] HTTP/1.1
Cookie: X-API-KEY=