A variable quantity is a quantity that can change or vary, such as the temperature, time, or the price of a product. These quantities can take on different values and are often represented by symbols or letters in mathematical equations. On the other hand, a non-variable quantity is a quantity that remains constant and does not change, such as the speed of light or the number of sides in a triangle. Non-variable quantities are fixed and do not depend on any other factors. **

Variable definition is the process of declaring a variable and specifying its data type, such as int, float, or string. This tells the compiler or interpreter that a variable with a certain name and data type will be used in the program. Variable initialization, on the other hand, is the process of assigning a value to the variable for the first time. This can happen at the same time as the variable is defined, or at a later point in the program. Initialization gives the variable a specific value to work with, while definition simply sets up the variable's characteristics. **

To display a Python variable in the terminal, you can use the `print()` function. Simply pass the variable you want to display as an argument to the `print()` function. For example, if you have a variable named `my_variable`, you can display its value in the terminal by using `print(my_variable)`. This will output the value of the variable to the terminal for you to see. **

You can display a PHP variable in real time by using AJAX (Asynchronous JavaScript and XML) to send a request to the server and update the content of a specific HTML element with the value of the PHP variable. This can be achieved by creating a JavaScript function that makes an AJAX request to a PHP script that returns the value of the variable, and then updating the HTML element with the received value. This allows the variable to be displayed in real time without the need to refresh the entire page. **

The variable display timeout on Android can be set by going to the "Settings" app, then selecting "Display" or "Display & Brightness" depending on the device. From there, you can find the option to adjust the screen timeout or sleep timer. This allows you to set the amount of time the device will wait before turning off the display when it's not in use. You can choose from preset options or set a custom time that suits your preferences. **

One can express a variable in terms of another variable by manipulating equations or formulas to isolate the desired variable on one side of the equation. This can involve performing algebraic operations such as addition, subtraction, multiplication, or division to rearrange the equation. By doing so, the variable of interest can be written in terms of the other variable, allowing for a clearer understanding of the relationship between the two variables. **

To store a JavaScript variable in a PHP variable, you can use AJAX to send the JavaScript variable to a PHP script on the server. The PHP script can then receive the variable using $_POST or $_GET superglobals and store it in a PHP variable. This way, you can pass data between JavaScript and PHP seamlessly. **

In Arduino, you can write a variable in a variable by using the concept of pointers. You can declare a pointer variable and then assign the address of the original variable to the pointer variable. This allows you to indirectly access the original variable through the pointer variable. Here's an example of how to write a variable in a variable in Arduino: ```C int originalVariable = 10; int *pointerVariable = &originalVariable; // Assign the address of originalVariable to pointerVariable *pointerVariable = 20; // Write a new value to originalVariable through pointerVariable ``` In this example, the value of originalVariable is changed to 20 by writing to it through the pointerVariable. **