How to create a user dictionary in eclipse?

The user dictionary is just a plain text file with one word per line.

You do not have to create this file, just put the path to where you want to store the file in the 'User defined dictionary' configuration that Eclipse shows you and it will create the file.

Yes, you can.

Create a text (.txt) file in a directory on your computer. Your Eclipse workspace may be appropriate. Add a new word on each line in the .txt file. You don't have to remember every word, that's what the help option is for, when you type a misspelling/a word eclipse doesn't know.

Go to Window → Preferences → General → Editors → Text Editors → Spelling and find the User defined dictionary section. To the right of that is a button called "Browse" click it, navigate to your text file, and choose it. You'll need to restart Eclipse in order for the changes to take effect.

You can also just type in the text box next to "User defined dictionary" in the Spelling Menu the path to where you want the file to be. It doesn't have to exist, but you must be familiar with paths if you want to do this.

When in C/C++, you must change the "Select spelling engine to use" option at the top of that options page to be "C/C++ spelling engine".