Function to grab Definition updates for Windows Defender. Felt a little messy how I used the foreach loop, but it works.

Get-DefinitionUpdates.ps1

Function WSUSUpdate {
<#
	Slight modification of https://gist.github.com/jacobludriks/9ca9ce61de251a5476f1
#>
    $Criteria = "IsInstalled=0 and Type='Software'"
    $Searcher = New-Object -ComObject Microsoft.Update.Searcher
    try {
        $SearchResult = $Searcher.Search($Criteria).Updates
        if ($SearchResult.Count -eq 0) {
            Write-Output "There are no applicable updates."
            exit
        } else {
	    $Session = New-Object -ComObject Microsoft.Update.Session
            $Downloader = $Session.CreateUpdateDownloader()
	    $Installer = New-Object -ComObject Microsoft.Update.Installer
            foreach ($i in 0..$SearchResult.Count) {
                try {
                    if (($SearchResult.Item($i).Title) -like 'Definition Update for Windows Defender - KB*') {
                        $Downloader.Updates = $SearchResult[$i]
                        $Downloader.Download()
                        $Installer.Updates = $SearchResult[$i]
                        $Result = $Installer.Install()
                        #$SearchResult[$i]
                    }
                }
                catch { }
            }
        }
    }
    catch {
        Write-Output "There are no applicable updates."
    }
}

WSUSUpdate
#If ($Result.rebootRequired) { Restart-Computer }