Learner's definition of EXTENSION
1 : the act of extending something: such as a : the act of making something longer or greater
[count] b : the act of straightening or stretching something (such as an arm or a leg)
2 [count] : extra time allowed for doing something 3 [count] : something (such as an interest or activity) that develops from something else — + of
◊ The phrase by extension is used to say that what follows comes from or is connected with something that has already been mentioned.
The new measures benefit taxpayers and, by extension, the economy.
He disliked authority and, by extension, all government officials.
4 a : a part that is added on to something to make it larger or longer b : extension cord 5 [count] : an extra telephone that is connected to the main line also : a telephone number that connects to a particular extension