Entanglement is at the heart of emerging technologies such as quantum computing, quantum sensing, and quantum networking. These technologies are developed by interdisciplinary teams of engineers, physicists, software developers, and other scientists. As a result, quantum concepts, such as entanglement, are essential to introduce to a wide range of disciplines. In response, many universities have implemented quantum information science and engineering (QISE) courses for undergraduate students. These QISE courses present quantum concepts to students from diverse disciplines at an introductory level, whereas previously, they were reserved for advanced physics courses. We interviewed students who had recently completed an introductory QISE course to investigate their conceptual and mathematical understanding of entanglement. We found that entanglement was most strongly associated with the idea of correlation between measurements, while only some students productively used the mathematical criterion of whether or not a quantum state was factorable.