Greenfield Project

In many disciplines a greenfield project is one that lacks constraints imposed by prior work. The analogy is to that of construction on greenfield land where there is no need to work within the constraints of existing buildings or infrastructure.

In software development, a greenfield project could be one of developing a system for a totally new environment, without concern for integrating with other systems, especially not legacy systems. Such projects are deemed as higher risk, as they are often for new infrastructure, new customers, and even new owners. For this reason, agile software development is often deemed the best approach, as it proposes how to handle those risks by developing small slices of complete functionality and getting them in the hands of customers (internal or external) quickly for immediate feedback.

source: https://en.wikipedia.org/wiki/Greenfield_project

Additional reading: Software Development: Greenfield vs. Brownfield (it's short and makes a distinction between brownfield and "legacy code").