pmenu is described as 'Terminal fuzzy finder/dmenu-replacement' and is an app. There are more than 10 alternatives to pmenu for Mac, Linux, Windows and Vim. The best alternative is fzf, which is both free and Open Source. Other great apps like pmenu are fzy, icepick, Peco and Percol.
pmenu alternatives are mainly File Search Utilities but may also be Terminal Emulators. Filter by these if you want a narrower list of alternatives or looking for a specific functionality of pmenu.