optical illusion noun
plural ∼ -sions [count] : something that looks different from what it is : something that you seem to see but that is not really there
▪ The closer building looks larger than the farther one, but it's just an optical illusion. The two buildings are actually the same size.
▪ The closer building looks larger than the farther one, but it's just an optical illusion. The two buildings are actually the same size.
