"Integrating the curriculum is the linking or weaving together of subject areas/disciplines to meet objectives required by state and national standards. It is a way of managing instruction whereby required objectives, skills, topics or themes are connected across disciplines" - Topics in Education: Integrated Curriculum
In short, when you are creating lessons and units that revolve around different disciplines while still incorporating the same theme and/or topic you are creating an integrated lesson plan. Integrated curriculum is also often referred to as: interdisciplinary curriculum, and thematic curriculum.