In Ambient Intelligence (AmI) scenarios, such as smart homes and similar assistive environments, system designers, medical personnel and - in general - non technically skilled people need to specify what to monitor with respect to occurrences of events and human activities. A Situation Description Language, called SDL, is introduced that allows to specify activity recognition templates as simple programs made up of formulas, and is provided with suitable tools to translate programs into symbolic structures maintained within an Ontology. Once encoded, formulas originate classification procedures that operate on actual sensory data. The focus of this paper is on formal specification of SDL formulas and corresponding translation within the Ontology. Examples are reported that demonstrate the benefits on context modeling introduced by the adoption of SDL.