Least Squares

What does “least” mean in “Least Squares”?

library(animation)
ani.options(interval = 0.3, ani.height = 450, ani.width = 600, 
    title = "Demonstration of Least Squares (Slope)", 
    description = "We want to find an estimate for the slope
                   in 50 candidate slopes, so we just compute the RSS one by one. ")
ani.start()
par(mar = c(4, 4, 0.5, 0.1), mgp = c(2, 0.5, 0), tcl = -0.3)
## slope changing
least.squares()
ani.stop()
 
## or with intercept changing
ani.options(interval = 0.3, ani.height = 450, ani.width = 600, 
    title = "Demonstration of Least Squares (Intercept)", 
    description = "We want to find an estimate for the intercept
                   in 50 candidate intercetps, so we just compute the RSS one by one. ")
ani.start()
par(mar = c(4, 4, 0.5, 0.1), mgp = c(2, 0.5, 0), tcl = -0.3)
# slope changing
least.squares(ani.type = "intercept")
ani.stop()