You may have said it best in your question: in words. After all, that's how several website tutorials describe it:
This is the line underneath "Pay to the order of" where you write out, in words, the dollar amount of the check.
Amount of check in words -- Enter the amount of the check in words. Start writing at the far left side of the line. Follow the dollar amount by the word "and," then write the amount of cents over the number 100. Draw a line from the end of the 100 to the end of the line.
...the different parts of the check (date, name and address of account holder, receiver of check or payee, amount in numerals, amount written in words, memo line, signature, account information)
Writing the amount of a check in words.
The amount written in words.
The amount in words is written on this line.
Amount of check: This should be written in words.
Term: written amount
Definition: amount of check written in words