"Proxy temperatures" refers to using indirect and natural indicators from the past, such as ice cores or tree rings, to estimate historical temperatures before the invention of thermometers. Full definition
From these, proxy temperature reconstructions of the last 2000 years have been performed for the northern hemisphere, and over shorter time scales for the southern hemisphere and tropics. (en.wikipedia.org)
These include questions about the use of climate data from tree rings and whether proxy temperature data and instrumental data have been improperly combined. (blogs.nature.com)
Most of proxy temperature records are from tree - ring measurements, but additional evidence comes from glacier ice, speleothems [stalagmites, stalactites and related mineral formations], corals and lake sediments. (dotearth.blogs.nytimes.com)