documentclass[oneside, openany,12pt]{book}

usepackage{amsthm,amsmath,latexsym,amsfonts,mathrsfs,graphics,graphicx,amssymb}

usepackage[mathcal]{eucal}

usepackage{setspace,titlesec,float,indentfirst,tocloft}

usepackage[top=1in, left=1in, bottom=1in, right=1in]{geometry}

usepackage{etoolbox}

titleformat{chapter}[display] {normalfontLARGEbfseriescentering}{chaptertitlename thechapter}{20pt}{LARGE}

renewcommandcontentsname{Table of Contents}

renewcommand{cftchapleader}{cftdotfill{cftsecdotsep}}

renewcommand{cftchappresnum}{MakeUppercase{chaptername}~}

renewcommand{cftchapaftersnumb}{qquad}

renewcommand{cftchapfont}{bfseries}

renewcommand{cftchappagefont}{bfseries}

renewcommand{cftchapaftersnum}{:}

setlength{cftchapnumwidth}{5.8em}

renewcommand{cftsecindent}{6.1em}

setlength{cftsecnumwidth}{2.1em}

renewcommand{cftsubsecindent}{6.1em}

setlength{cftsubsecnumwidth}{2.1em}

theoremstyle{plain}

newtheorem{thm}{Theorem}[section]

newtheorem{mthm}[thm]{Main Theorem}

newtheorem{clm}[thm]{Claim}

newtheorem{lem}[thm]{Lemma}

newtheorem{cor}[thm]{Corollary}

newtheorem{rmk}[thm]{Remark}theoremstyle{remark}

newtheorem{exm}[thm]{Example}theoremstyle{definition}

newtheorem{case}[thm]{Case}theoremstyle{definition}

newtheorem{defn}[thm]{Definition}

begin{document}

chapter{CHAPTER WITH NO SUBSECTION}

defn blah blah blah

usepackage{chngcntr}
...
chapter{CHAPTER WITH NO SUBSECTION}
counterwithout{defn}{section}
defn blah blah blah