Retrieving and analyzing trajectories favor the study of human behaviors in space and time. Although Time Geography has long proposed a conceptual framework for the interpretation of trajectory data, there is still a need for database and semantic models that support representation and manipulation. In fact, the concept of trajectory is rarely addressed as a sui generis data type that can be embedded in a database structure. This paper introduces an algebraic Spatio-Temporal Trajectory data type (STT) for the representation of object trajectory. The STT is an abstract data type endowed with a set of operations designed as a way to cover the syntax and semantics of a given trajectory. It is considered as a data structure that can be used for the design and implementation of spatio-temporal databases.