I haven't found anything related to this device neither on internet nor in dictionaries
It is an equipment used to make up for the problem of not having enough sockets, nearby.
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There are a lot of different ways to talk about this item. Here are a few ways I've heard it spoken about:
Power strip - like the item you have pictured, it has multiple outlets on it and plugs into the wall, and usually contains a power switch.
Extension cable/cord (according to @Canadian Yankee, an extension lead in the UK) - kind of similar to a power strip, but it only has one outlet and usually a longer cable, and usually doesn't have an on off switch. This is different from the item pictured, but is still used to bring power to a place where there is none.
Outlet strip - just another way to say power strip.
Surge protector - though this looks the same as a power strip/outlet strip, a surge protector has special circuitry to protect your devices against a power surge. Though it's a specific type of power/outlet strip, people usually use them interchangeably these days, because many people don't know the difference.
Bear in mind, I'm an American English speaker in the Pacific Northwest, so those are the terms we use out here. Other people may have other ways of saying it, but if you use one of those three properly, people will understand what you mean.
Yes, the image you attached would indeed be called a "power strip".
A "power outlet" would be a socket for power plugs that is attached to the wall or some similar surface (floor, etc).
Alternatively, you may call one of the sockets in the power strip a "power outlet", as in "Are there any [power] outlets left in the power strip that are still empty? I need to plug in my charger."
What you have in the picture is a power strip or a power bar.
A power outlet, power point, outlet, receptacle, or socket is a device in a wall that you put a plug into in order to connect electrical equipment to the power supply of a building. (Oxford Dictionary). It can also refer to any of the individual sockets on a power strip.