The term 'ONE THOUSANDTH' can be represented by the word 'MILLI', which is a five-letter word. The prefix 'milli-' is derived from the Latin word for thousand, and it is used to denote one thousandth of a unit. It is commonly used in the metric system to represent fractions of a meter, gram, or liter. In other contexts, it can also be used to represent one thousandth of an hour, minute, or second.