apt install : command not found

[lists . debian]

Hello to all, After this morning I realize that command "apt install" isn't 
working anymore on my PC. I already used a few days ago... I've tried several 
other options (always with sudo): apt update apt search ______ apt upgrade apt 
show ______ ... And they all work as intended, only "apt install" is giving me 
the output "command not found" :( Do you have any idea on what might be the 
problem? Thanks all in advance for the help, Best regards, Marc