Dictionary.net

• a layer in the stratosphere (at approximately 20 miles) that contains a concentration of ozone sufficient to block most ultraviolet radiation from the sun

2006 - WordNet 3.0
By Princeton University

• a layer in the stratosphere (at approximately 20 miles) that contains a concentration of ozone sufficient to block most ultraviolet radiation from the sun

2011 - English Dictionary Database
By DataStellar Co., Ltd