To make a pledge to (someone); to promise, guarantee (someone of something); to assure.
"She ensured him that his promotion was secure before he left for the interview."
In plain English: To ensure means to make sure something definitely happens or is true.
"Please ensure you lock the door before leaving the house."
Usage: Use "ensure" to mean making certain that something happens or is done correctly, such as ensuring the lights are turned off. Do not use it to mean promising someone that you will do something, which requires the word "assure."
The word ensure comes from the Old French word seur, meaning "sure," which entered English through Anglo-Norman and Middle English. It is a doublet of the related word insure, sharing the same root but developing distinct spellings and nuances over time.