气凝胶（Aerogel），也称作空气胶或是稀密封，是世界上密度仅次于全碳气凝胶的人造发泡物质。它的制造是将气体取代液体在凝胶中的位置而成，而如此做出的结果造就了拥有数种杰出特性的极轻物质。其中最引人注目的是它良好的隔热能力。这样的物质拥有许多俗名与暱称，如：冻结的烟雾（frozen smoke）、固态的烟雾（solid smoke）、固态的空气（solid air）、蓝烟（blue smoke）、旧金山之雾（San Fransisco fog）等，而这些都源自于他的透明性与物质中的光线散射能力。不过，这种物质的触感却像是聚苯乙烯一般。

Aerogel is a synthetic porous ultralight material derived from a gel, in which the liquid component of the gel has been replaced with a gas. The result is a solid with extremely low density and low thermal conductivity. Nicknames include frozen smoke, solid smoke, solid air, or blue smoke owing to its translucent nature and the way light scatters in the material. It feels like fragile expanded polystyrene to the touch. Aerogels can be made from a variety of chemical compounds.