According to PMBOK® Guide,“A project is a temporary endeavour undertaken to create a unique product, service or result.”

You could elaborate more, as one document I found does, to further explain temporary, unique, and project, service or result.

The important elements of this definition are temporary, unique, and project, service or result. Projects are temporary in nature because they have definitive start and end points; without these limits the work is day-to-day operations. What makes a project unique is that it is something new, something the organization has not previously done in this particular way. All projects must produce something: a Product, Service or Result.

A project can be

• New Service / Product launch
• Merger and acquisitions
• New Business process implementation
• Infrastructure or building construction

In other words, projects are how organizations change, grow and innovate.