What is a Subquery?

By the definition "Subquery or Inner query or Nested query is a query in a query."

SELECT studentName, major, studentId
FROM StudentTable
WHERE major IN 
 (SELECT major FROM MajorTable WHERE major = 'Awesome')

SELECT *
FROM (SELECT studentId FROM StudetTable WHERE studentId < 3)

declare @studentName varChar(50)
set @studentName = (SELECT top 1 studentName FROM StudentTable)