An indefinite ordinal is a type of number that is used to indicate an unspecified position in a sequence. It is usually represented by the letter 'N' followed by 'th', such as 'NTH'. This type of number is commonly used when referring to an unknown or unspecified order, such as in the phrase 'the NTH time'. It can also be used to refer to a specific item in a list without specifying its exact position.