Computer

an electronic device that is not necessarily designed for just a small amount of tasks, but for any number of programmable tasks.

for example, a cell phone would not be considered a computer, since it can only make phone calls and send SMS text messages, but a smart phone might be considered a computer, because it can run any number of apps that can make use of the smart phone's hardware to do things the phone was not necessarily designed for.

realistically speaking, a computer has all of the following parts:

• mainboard (motherboard): connects all other parts to help them work together
• processor (CPU): the "brain" of a computer
• temporary memory (RAM): stores currently running processes until done
• permanent storage(hard drive, etc.): keeps files needed for system startup, and personal use
• input devices (keyboard or mouse): used to enter data and navigate through the various areas of the computer on-screen (Note: on smart phones, the touch screen would be considered a type of input device)
• display (screen or monitor): some way to see what is happening so that you can react accordingly
• power supply (PSU): the internal hardware that takes in AC voltage and uses it to power the computer and all it does

that is pretty much all you need to have a working computer, other than some sort of Operating System (such as Windows or Android).