There are couple of definitions for architecture.
The most common and used ones are :
1- The fundamental concepts or properties of a system in its environment embodies in its elements, relationships, and in the principles of its design and evolution.
Source : ISO 42010:2011
2- An architecture is the set of significant decisions about the orgranization of a software system, the selection of the structural elements and their interfaces by which the system is composed, together with their behaviour as specified in the collaborations among those elements, the composition of these structural and behavioural elements into progressively larger subsystems, and the architectural style that guides this orgranization – these elements and their interfaces , their collaborations, and their composition.
Source : Grady Booch / UML Definitions 1999.