What is the name of this Nintendo Switch accessory?

The accessory that ships with every Nintendo Switch is called the Joy-Con Grip. It's only purpose is to hold the Joy-Cons, in case you want to use them as a traditional controller. It does not charge the Joy-Cons at all. The Joy-Con Grip ships with every Nintendo Switch console (except the Switch Lite), and cannot be purchased separately.

The Joy-Con Charging Grip, meanwhile, is a separate purchase (at least for the launch Switch).

You can tell both grips apart by the fact that the Charging Grip is partially transparent: ^{Picture taken from Polygon}

This accessory is simply called Joy-Con Grip.

https://www.nintendo.com/switch/buy-now/accessories/