light hour - the distance light travels in a vacuum in one hour; approximately one billion kilometers. 1  astronomy unit - a linear unit used for astronomical distances. 2  light year, light-year - the distance that light travels in a vacuum in 1 year; 5.88 trillion miles or 9.46 trillion kilometers.
keystone. n. 1. (Building) Also called: headstone or quoin the central stone at the top of an arch or the top stone of a dome or vault.2. something that is necessary to connect or support a number of other related things.(ËkiËstoÊn). n.eystone. n. 1. (Building) Also called: headstone or quoin the central stone at the top of an arch or the top stone of a dome or vault.
A keystone species is one that other lifeforms in an ecosystem  depend on. Mosquitoes are a keystone species because they are  extremely low on the food chain, and if they w â¦ ere removed from an  ecosystem, the food supply would be interrupted.hey are critical factors in ecological processes such as â¦ energy flow and nutrient cycling.If we can identify a keystone species and quantify its impact on its community then we can usually understand how the community functions as a whole.