In reality, all of us who watched tokusatsu didn't knew the meaning of "Henshin" on the first time we hear it, with exceptions to Japanese people who, indeed already know about this stuff.

Henshin, literally "to transform or change", is the words tokusatsu heroes, and in some extent, sometimes present in anime, used to transform to his/her alter-ego. It sometimes shows the actor wearing some kind of armor from nothingness. This word is mostly often said or used in the Kamen Rider Series everytime they transform as a Kamen Rider.

There are also some variations in saying "Henshin" outside Kamen Rider Series. This includes "souchaku" and "chakusou" which literally means "to equip". These are being used by the Chou Seishin Series and Tomica Heroes Series respectively. Super Sentai Series uses different variations that is relevant to their plot and story. Examples includes "BakuryuChange" (Abaranger), "Emergency Dekaranger!" (Dekaranger), "Maji Maji Majiro" (Magiranger), "Beast On!" (Gekiranger) and "Ippitsu Soujo" (Shinkenger) pls many more!

You need a certain item to be able to henshin. These are called "Henshin Items" (isn't that obvious?). Different series have different henshin items. This includes belts/drivers (Kamen Rider Series), bracers and cellphones (Super Sentai) and many more!

Would you believe it? The word "Henshin" already reached American cartoons. One example is Capcom's character, Viewtiful Joe. He shouts the phrase, "Henshin a go-go, baby!" when he transforms into his comic book style alter-ego. Many have said that Viewtiful Joe is widely inspired by the Kamen Rider Series.