Theta (θ) is the eighth letter of the Greek alphabet and a symbol commonly used in math. It doesn't have a specific meaning, but is used as a substitute variable with angles and polar coordinates. Basically, its meaning changes depending on the context in which it is used. If you're struggling to find the theta sign on your TI-84 Plus graphing calculator, don't worry. It's definitely there – it's just not immediately obvious.
Turn on Your TI-84
Press the On key on your TI-84 calculator. An information box will appear. Press the 1 key to continue to the home screen and not see the information box the next time you turn it on, or press the 2 key to simply continue to the home screen.
Set Polar Mode
To get the theta sign, your TI-84 calculator needs to be in Polar mode. Press the Mode key (near the top of the keyboard). By default, the TI-84 is in Normal mode. To set Polar mode, press the downward-pointing triangle key until you get to the line reading "Function Parametric Polar SEQ." Next, press the right-pointing triangle key until Polar is highlighted. Press Enter to select this mode. (In Polar mode, r is a function of θ.)
Sciencing Video Vault
While your TI-84 is in Polar mode, press the [X,T,θ,n] key (just below the Mode key) to select and insert θ, together with any other characters you require for your expression.
TL;DR (Too Long; Didn't Read)
You can enter and use several types of data, including real and complex numbers, matrices, lists, functions, stat plots, graph databases, graph pictures and strings, on your TI-84 graphing calculator. It uses assigned names for variables and other items saved in its memory, and you can also create your own five-character names for lists. For real numbers (including fractions) and complex numbers, θ can be used in the name. To store a variable name, enter the value you want to store on a blank line, and then press the key with "sto" and a right-pointing arrow on it. Next, press the Alpha key and then the letter of the variable you want to store the value to. Press Enter to store the value to the variable.
To ensure variables don't get deleted, you can archive them in a protected area of memory space. This lets you safely store variables you don't need to edit frequently and free up memory space for other applications.