How to replace current word under cursor in Emacs

How do I replace current word under cursor in Emacs? I know that I can use query-replace or replace-string but every time I do so I have to type entire string to be replaced, this is just annoying. Vi has equivalent command cword and I can use a shortcut to pull the word under cursor for replacement: nmap z :%s#\=expand("")\># Anybody know how to do it with Emacs?