Conceptual models must be expressed in a suitable language in order to communicate them. To avoid misunderstandings, this language should allow for formulating clear, unambiguous expressions. First order logic is a language that has all the properties one needs and wants, but it has the drawback that its common Peano-Russell notation puts off people who lack a background in logic and mathematics. The DEMO Specification Language (DEMOSL) offers a user-friendly look, although it is firmly based on first order logic. One produces precise formulations that resemble English sentences. The syntax of DEMOSL is defined in the so-called Extended Backus-Naur Form (EBNF). In addition to (formal) textual expressions, it allows for (formal) graphical representations. Except for the Action Model, one can represent the DEMO aspect models in diagrams. DEMOSL comprises specific diagrams for each of the other models: the Cooperation Model, the Process Model and the Fact Model. The diagrams are kept simple, which means that what cannot be expressed in a diagram, must be expressed in (formal) text. In addition to the explanation and illustration of DEMOSL, the meta model or schema of each of the four aspect models is presented and discussed, as well as of each of the kinds of diagrams and tables. They are also expressed in DEMOSL.