Glacier retreat means that glaciers are getting smaller over time. The ice is melting and shrinking, causing the glacier to move back and get smaller. Full definition
Over long periods, glacial response to climate change becomes obvious as glaciers retreat and, in some cases, disappear. (nsidc.org)
But Chinese scientists warn that global warming is main cause of glacier retreat and the tourism ban will do little to reverse the trend. (earthjournalism.net)
From 1850 - 1945 worldwide glacier retreat occurred as the Little Ice Age ended. (illconsidered.blogspot.ru)