DOS

1. disk operating system -- a kind of operating system for computers?.

2. [denial of service]? -- often DoS to distinguish it from the first sense; in the context of computer networking, the situation where a system is attacked in such a manner that "normal" network communication is blocked by excessive traffic, or the system itself is rendered nonfunctional due to overload or caused to crash; distinct from a compromised system, which may continue to operate but is put to subversive use or exposed to illicit monitoring

3. In condensed matter physics a common acronym for density of states. The density of states, N(E), for electronic energy levels in a solid is defined as follows: N(E)dE is the number of allowed energy levels per unit volume of the solid, in the energy range E to E+dE.