How to include search results where keyword input exists only as a category title?
It's hard to say how you could include it since we don't know the rest of your search. Generally the best idea to search is with php and not with twig but you could grab all categories with the title (or whatever field it should be) with

{% set categoryIds = craft.categories.title("Your search term").ids() %}


Then you can fetch all entries that are related to these categories with

{% set entries = craft.entries.relatedTo(categoryIds).find() %}


However: if you just follow this example category titles are included in your search.