process or manner of functioning or operating
"the power of its engine determines its operation"
"the plane's operation in high winds"
"they compared the cooking performance of each oven"
"the jet's performance conformed to high standards"
Action of the verb function.
"The machine stopped functioning after running out of fuel."
In plain English: Functioning is how well something works properly without breaking down.
"The functioning of this engine is smooth and reliable."
present participle of function
"The malfunctioning machine stopped functioning properly after the power surge."
In plain English: To function means to work properly and do what you are supposed to do without breaking down.
"The old car is still functioning well after all these years."
Functional, working
"The old laptop was still functioning perfectly after all these years."