up vote 3 down vote accepted. $\pi$ is the name of the constant relating the diameter and circumferance of a circle. It's a definition, a particular constant that we thought deserved a name. $e$ happens to be the name of a constant from a particular limit. Like $\pi$, we named it because we thought it would be useful. In Physics, many, many constants have names. Gravitational constants, expansional constants, electrical constants, etc. Each is useful, but somewhat arbitrarily chosen.
Bacteria: The Monera Kingdom Bacteria are classified into two groups: Eubacteria (true bacteria) and Archaebacteria (Ancient Bacteria). They are autotrophic. 28-1 Chapter 28: Microbiology. 28-2 Viruses Viruses are noncellular, nonliving particles and therefore are not included in the classification scheme.
In this tree of life, one of the basal branches evolves into a group of organisms that is part of Domain Prokarya, and is the current extant Kingdom Bacteria. Eubacteria is a synonym used by some authors for the kingdom of the bacteria. As you may recall, bacteria were first part of Kingdom Plantae (green) when there were only two kingdoms. The organisms had cell walls at least and so that was enough for them to end up in that kingdom. Once we understood that they lack a nucleus or chloroplast or mitochondrion, the bacteria needed to be moved into a new kingdom...Monera (yellow) was created for them in the 3 to 5 kingdom era. The Archaea were split out from Monera in the transition to 6 kingdoms, so Monera was renamed Bacteria (or Eubacteria).