Add word to TexMaker dictionary?

If adding is what you just need then this answer might work for you. If you want to install a new dictionary, follow the link in the comment. With your terminal, cd to the directory of your TeXMaker dictionary. If you don't know where, in TeXMaker click

Options > Configure TeXMaker > Editor

You can see the directory of the dictionary in the option Spelling dictionary. In Ubuntu, once you are in the directory, type

sudo gedit en_GB.dic

and put the word you want to add in there.

In Windows, just go to the directory and double click on the file or open with Notepad. Then write the word you want to add.

You might also want to save the customized file in another folder and have your Spelling dictionary option point to the custom file.

In the newer versions they have also added a "Spell check" feature (Control + Shift + F7 ; or from the menu: Edit > Check Spelling) which allows you to add new words by clicking "always ignore".

The custom words are stored in $HOME/.config/xm1/texmaker.ini (unix) or in C:\Users\[username]\AppData\Roaming\xm1 (windows)

In version 4.1.1, you can right click a word underlined in red and then click "Check Spelling Selection". That will open a dialog box where you need to click "Always Ignore".