used when telling someone the last thing at the end of a list or a series of statements [≠ firstly]:
Lastly, could I ask all of you to keep this information secret.
WORD CHOICE: WORD CHOICE: lastly, finally, eventually, in the end, at last!! Do not use lastly to say what happened at the end of a period of time or after several other things happened. Use one of the following expressions:Use finally or eventually to say that something happens after a long time• Finally we managed to get the car to start. • When she eventually turned up, the food was cold.Use eventually or in the end to say what the result or outcome of something was• They eventually got bored and went home. • In the end we decided to cancel the trip.Use at last to say that something happens after a long period of waiting or trying, when you are glad about this• It's good to be home at last. • At last, the pizza's here!Use lastly or finally to introduce the last point you want to make, the last action in a series of actions, or the last item in a list• Lastly, I would like to remind you that smoking is not allowed. • Load the paper, select the number of copies, and lastly press 'Print'. • You add flour, salt, and finally milk.
Definition from the Longman Dictionary of Contemporary English Advanced Learner's Dictionary.