documentclass[11pt]{article}

% Necessary packages
usepackage{amsthm}
usepackage[most]{tcolorbox}

% Problem Packages
usepackage{parskip}
usepackage{hyperref}
usepackage{cleveref}

theoremstyle{definition}
newtheorem{definition}{Definition}

% Coloured box around definitions
tcolorboxenvironment{definition}{
    sharp corners,
    frame hidden,
    breakable,
    enhanced,
    colback=gray!12,
    parbox=false
}

begin{document}
    begin{definition}[Hilbert Space]
        An inner product space $X$ is said to be a emph{Hilbert Space} if it is complete with respect to the induced metric.
    end{definition}
end{document}