Dissimulation is a form of deception in which one conceals the truth. It consists of concealing the truth, or in the case of half-truths, concealing parts of the truth, like inconvenient or secret information. Dissimulation differs from simulation, in which one exhibits false information. Dissimulation commonly takes the form of concealing one's ability in order to gain the element of surprise over an opponent.
Francis Bacon has the following to say on the distinction between simulation and dissimulation, "Dissimulation, in the negative; when a man lets fall signs and arguments, that he is not, that he is... Simulation, in the affirmative; when a man industriously and expressly feigns and pretends to be, that he is not." The essay from which this comes can be found here.
In software development, dissimulation is the logical converse of simulation. In this context dissimulation is a worthwhile cryptographic tool for protection against a differential attack, implemented by having semantically equivalent executions look different. This goal is to prevent automated tools from detecting and distinguishing any meaningful differences between such executions.
Pool hustling is a form of dissimulation, because the hustler conceals his real talent. It is sometimes considered a form of simulation because every hustle conveys false information about the hustler's abilities but this is incorrect. A hustler gives the false appearance that he isn't something. Simulation would be giving the false appearance that you are something. A hustler is a 'dissimulator'. An equivalent 'simulator' would be a man pretending (by his confident movements and his bragging) that he was excellent at pool, when in fact he was terrible (he can't make a single shot).
In the animal kingdom a predator giving the impression that it is harmless is an example of dissimulation, such as the leaf fish. An animal that is typically prey giving the impression that it is dangerous is an example of simulation, such as the frill-necked lizard.
Dissimulation and hypocrisy from The Summa Theologica of St. Thomas Aquinas. Note, however, that the Latin original concerns "simulatio," which is different from "dissimulatio." ST II.II.110.3ad4 mentions dissimulation properly speaking.