aviket

This is a short and sweet autocpmplete plugin, useful for autocompleting 
words, as the post author types the contents in the wordpress content area. 
By default, “wordpress” is the only word, which is autocompleted. So after 
installing and activating the plugin, when you, as an author, start typing 
the blog contents, after adding a new post, and when when you type “wo” , 
a small autocomplete box pops up beneth, with “wordpress” written on it. 
If you want to type “wordpress”, you can click on that widget, or if you 
want to type “wolverine” , you can keep typing, ignoring the widget. 
For adding more words, go to settings=>autocomplete options 
In the top row, there is a text box and a button. Type the word you want to 
add and press the “add word” button. This text box accepts only alphanumeric inputs. 
Now the new word is added in the list. To reset the list, press the “reset” button. 
And the list now resets to the original value “wordpress” 
This plugin uses “textcompleter” Jquery plugin by “Yuku Takahashi” 
https://github.com/yuku-t/jquery-textcomplete

Autocomplete

This is a short and sweet autocpmplete plugin, useful for autocompleting

jojaba

This plugin is meant to add the word autocomplete in search input field by using the index stored in Relevanssi database. So you have to <a href="https://wordpress.org/plugins/relevanssi/" rel="ugc">install first Relevanssi</a> (free or premium version) to have it to work.
The JS script used to accomplish the autocompletion work has been developed by <a href="http://lea.verou.me/" rel="nofollow ugc">Lea Verou</a> and is called <a href="https://leaverou.github.io/awesomplete" title="Go to the Awesomplete homepage" rel="nofollow ugc">Awesomplete</a>. It is very simple to set up, lightweight and accessible.
<h4>List of functionalities</h4>
<ul>
<li>Settings available: styling, min characters typed to show the autocompletion box, max suggestions in autocompletion box.</li>
<li>Get the Relevanssi Indexed words to have the autcompletion list</li>
<li>Fast and lightweight</li>
<li>English and french languages included</li>
</ul>

Autocomplete For Relevanssi

Relevanssi author here. Please do not use this plugin in its current form. It's dangerous for performance and may cause server crashes. It will also slow down page loads in a rather terrifying manner.

Dangerous, do not use

Autocompletion functionality for WordPress search input when Relevanssi plugin is installed.

Autocomplete vs Autocomplete For Relevanssi