"Top up" or prepaid mobile phone credit, is known elsewhere in the world as:

• Credit
• Reload
• Recharge
• Airtime - India
• Minutes - USA
• Load - Philippines

Top up credit in Thailand are the funds added to a pre-paid phone to allow you to make phone calls, send SMS and use data. As inbound calls are free, you don't need credit to receive calls. Prepaid can also be called “Pay as you go” or PAYG by some people.

What are "top up online" or "refill on mobile"?

These are trade names for top up credit sent from a merchant via their phone to another phone by entering in their phone number and the amount of the top up. A confirmation SMS is sent to both phones when the transaction is complete.