This Perspective discusses the challenge of defining cities and the need for a universally accepted method to delineate them. Researchers have used various criteria and datasets, including population density and traffic flows, to define cities. However, a more comprehensive approach is needed, one that considers both the physical and functional aspects of urban areas. The authors argue that cell-phone data, which reflect people's presence and movement, can serve as a proxy for defining cities. These data, combined with computational methods, can support dynamic and tailored city delineations that meet different research needs. Historically, cities have evolved from small settlements around marketplaces, growing as transportation and technology advanced. Today, cities are increasingly interconnected, with populations spreading into their hinterlands, blurring the line between urban and rural areas. Existing definitions of cities, based on administrative boundaries or physical features, face challenges in providing consistent, comparable results across regions and countries. The paper outlines six criteria for a good city definition: clarity and ease of understanding, universality and accessibility of data, consideration of both form and function, careful parameter selection, validation through quantitative criteria, and compatibility with administrative boundaries. It also highlights the limitations of current definitions, including arbitrary parameters and the difficulty of cross-regional comparisons. The authors propose a feasible process for city delineation using cell-phone data. This involves calculating population density, deriving movement patterns, setting thresholds based on critical characteristics of urban systems, and validating results through multiple criteria. Cell-phone data offer the advantage of capturing both form (presence of people) and function (movement flows), making them a valuable tool for defining cities. The paper emphasizes the importance of using standardized, globally accessible datasets to define cities, as this would facilitate comparative studies and support urban science research. While cell-phone data have limitations, such as access issues and privacy concerns, they provide a consistent and detailed view of urban areas. The authors call for a collaborative effort to develop standardized city definitions based on cell-phone data and other global datasets, to better address urban challenges worldwide.This Perspective discusses the challenge of defining cities and the need for a universally accepted method to delineate them. Researchers have used various criteria and datasets, including population density and traffic flows, to define cities. However, a more comprehensive approach is needed, one that considers both the physical and functional aspects of urban areas. The authors argue that cell-phone data, which reflect people's presence and movement, can serve as a proxy for defining cities. These data, combined with computational methods, can support dynamic and tailored city delineations that meet different research needs. Historically, cities have evolved from small settlements around marketplaces, growing as transportation and technology advanced. Today, cities are increasingly interconnected, with populations spreading into their hinterlands, blurring the line between urban and rural areas. Existing definitions of cities, based on administrative boundaries or physical features, face challenges in providing consistent, comparable results across regions and countries. The paper outlines six criteria for a good city definition: clarity and ease of understanding, universality and accessibility of data, consideration of both form and function, careful parameter selection, validation through quantitative criteria, and compatibility with administrative boundaries. It also highlights the limitations of current definitions, including arbitrary parameters and the difficulty of cross-regional comparisons. The authors propose a feasible process for city delineation using cell-phone data. This involves calculating population density, deriving movement patterns, setting thresholds based on critical characteristics of urban systems, and validating results through multiple criteria. Cell-phone data offer the advantage of capturing both form (presence of people) and function (movement flows), making them a valuable tool for defining cities. The paper emphasizes the importance of using standardized, globally accessible datasets to define cities, as this would facilitate comparative studies and support urban science research. While cell-phone data have limitations, such as access issues and privacy concerns, they provide a consistent and detailed view of urban areas. The authors call for a collaborative effort to develop standardized city definitions based on cell-phone data and other global datasets, to better address urban challenges worldwide.