First Update the system. Then try with commandline as follows:

sudo apt-get update && sudo apt-get install steam

Steam can be install as a flatpak package in all currently supported versions of Ubuntu. Open the terminal and type:

sudo add-apt-repository ppa:alexlarsson/flatpak # required in 16.04 only, not in later releases
sudo apt update 
sudo apt install flatpak
sudo flatpak remote-add --if-not-exists flathub 
flatpak install flathub com.valvesoftware.Steam

Steam can be launched by clicking on its icon in the Dash or launched from the terminal with the following command:

flatpak run com.valvesoftware.Steam 

Many games require an online purchase, but some popular games, such as Team Fortress 2, are free to play. When searching in the store, be sure to narrow results by the SteamOS/Linux operating system. Not all Linux games are compatible with your system, so make sure you check the requirements before purchasing games.

The steam package is part of the "multiverse" repository for Ubuntu versios 14.04 onwards, but only for 32 bit versions of Ubuntu. If you want the 64-bit Steam package, you need to add the Steam repository to your sources.

32-bit Ubuntu

• enable the "multiverse" repository
• run sudo apt-get update
• Install steam using either apt-get or the Software Center.

64-bit Ubuntu

• add the following line to /etc/apt/sources.list:

  deb precise steam

• run sudo apt-get update && sudo apt-get install steam-launcher

or

• install the .deb manually from the official site.

If it is not possible to find in the Software Center and not possible to install .deb file, then add the source in the software center manually.

Source = deb  precise steam

Now possible to install it !

sudo apt-get install steam

Using Xubuntu 14.10 x64, from the terminal:

sudo apt-get update && sudo apt-get install steam

This will download and install Steam. Accept the EULA, update Steam, then sign in with your Steam login. BAM! Steam on Xubuntu!

I confirm that using

sudo apt-get install steam

works fine (still I don't know why steam seems not available on Software Center), however if the application does not appear in menu it is not necessary rebooting, just run

sudo updatedb

You will need to install the Steam package manually.

Go to and choose the "Install Steam now" link. This will download a debian package (ends in .deb) which you can open directly in the Ubuntu Software Center app. The "Install" button will then install Steam.

It appears it does not show up in the launchbar, but you can search for it (tap the Windows button) and launch it. When launched right-click on the icon to lock it to the launch bar. You can then search for the Steam

If the apt-get install steam doesn't work do the followings

wget 
sudo dpkg -i steam.deb

you may have to rm -r ~/.steam ~/.local/share/Steam directory for removing the old config

If you have problems installing or running it, try following question