top-up

1. not comparableThat serves as an addition

   English university students will have to pay top-up fees.

plural top-ups

1. countableuncountableAn addition.

   Energy will be generated from solar panels, water will be recycled and extracted from soil and the astronauts will grow their own food - they will also have an emergency ration and regular top-ups as new explorers join every two years.

   2013, Melissa Hogenboom, “Applicants wanted for a one-way ticket to Mars”, in BBC News

2. countableuncountableA serving of drink used to top up an existing glass.
3. countableuncountablebusinessAn additional premium paid over the initial premium in order to increase benefit values.
4. countableuncountableAdditional credit purchased for a mobile phone.

   I added a $20 top-up on my cellphone's data plan.

5. countableuncountablemedicineA dose of epidural anesthetic added to previously injected spinal anesthetic in combined spinal-epidural anesthesia

   The reinforcement of anesthesia by an epidural “top-up” in combined spinal-epidural anesthesia may be explained by a dual mechanism: a volume effect compressing the dural sac and a local anesthetic effect.

   1999, The Epidural “Top-Up” in Combined Spinal-Epidural Anesthesia: The Effect of Volume Versus Dose

6. countableuncountableeducationThe situation where a student who holds a qualification equivalent to part of a degree course is then accepted onto a degree course at an intermediate point, without having to start it from the beginning.