1.verbTo assess the similarities and differences between two or more things ["to compare X with Y"]. Having made the comparison of X with Y, one might have found it similar to Y or different from Y.
"Compare the tiger's coloration with that of the zebra."
2.verbTo declare two things to be similar in some respect ["to compare X to Y"].
"Astronomers have compared comets to dirty snowballs."
3.verb(grammar) To form the three degrees of comparison of (an adjective).
"We compare "good" as "good", "better", "best"."
4.verbTo be similar (often used in the negative).
"A sapling and a fully-grown oak tree do not compare."