ˈsərˌtaks, ˈsə̄ˌ- noun
Etymology: sur- + tax
: an additional or extra tax: as
a. : a special tax levied against certain classes of persons or goods over and above the general charge upon the whole group : an extra charge (as on a railroad for special accommodations)
b. : a graduated income tax in addition to the normal income tax imposed on the amount by which the net income of an individual exceeds a specified sum — called also additional tax ; compare supertax
c. : a supplementary tax added at a later date than the normal rate (as in customs duties)
d. : the charge made on a semipostal stamp above the amount required for postage