Molecules of only one kind

• Having only one component
• Having a single part
• Having one component
• Consisting of a single element or component
• Acting on a single number, mathematically
• Having but one component
• Having a single element
• Consisting of a single element

WordNet Word definitions in WordNet
adj. consisting of or involving a single element or component; "in a unary operation in a mathematical system one element is used to yield a single result"

Wiktionary Word definitions in Wiktionary
a. 1 Consisting of or involving a single element or component. 2 (context mathematics programming computer engineering English) Of an operation, function, procedure(,) or logic gate, taking exactly one operand, argument, parameter(,) or input; having domain ...