A variable is a word that Soulver will interpret as a number in all documents. Soulver has some built-in constants which you cannot edit, such as pi (3.14…) and e (2.71….). You can add new ones in the Numbers window.
Open the Numbers window (click the toolbar item, or go "Window" > "Numbers").
Make sure the "Variables" is selected on the left.
Click the plus button at the bottom of the window.
Double click in the name field and give the variable a name. You can use letters and numbers, but no operators (+, ^, etc) and no spaces.
Double click in the value field and give the variable a value.
You can write expressions here, such as 10+20, however Soulver will instantly calculate the answer and the variables value will become 30.
Select the variables you want to delete in the table, and press delete or click the minus button at the bottom of the window.
Once you delete a variable, you cannot undo.
Tips
It's handy to make a variable with the letter k as 1,000. Then you can type 1k in Soulver and get one thousand.
You can define a variable called "half" and set it to 0.5. Then you can do expressions like "half × 460".