Sometimes we say it with words
"As input values of 𝑓𝑥 decrease without bound, the output values decrease without bound"
"As input values of 𝑓𝑥 decrease without bound, the output values increase without bound"
"As input values of 𝑓𝑥 increase without bound, the output values decrease without bound"
"As input values of 𝑓𝑥 increase without bound, the output values increase without bound"