A website is a presence on the World Wide Web. As of 2017, there are more than one billion. To qualify as a bona fide site, it must be available over the Internet around the clock.
Tube means a hollow cylinder. An example of a tube is manicotti pasta. The definition of a tube is a pliable container with a screw cap at one end to keep it closed. An example of tube is the container that toothpaste comes in.
What is a centrifuge test tube? A tapered and specially-engineered test tube specifically for centrifuge work, designed to resist the high G-forces induced by the centrifugal forces.