noun ( BrE )
1.
a payment that you make to increase the amount of money, etc. to the level that is needed :
a phone top-up (= to buy more time for calls)
Students will have to pay top-up fees (= fees that are above the basic level).
2.
an amount of a drink that you add to a cup or glass in order to fill it again :
Can I give anyone a top-up?