The numbers 1²,4²,9²,16², 25² are called perfect squares
Because all of these integers have an integer as a square root as well.
1² =1
4²=16
9²=81
16²=256
25²=625
√1=1
√4=2
√9=3
√16=4
√25=5
This term goes back to Ancient Greeks when they use geometry to calculate the square root of these numbers as the sides of squares.