This mixed-methods study examines the conceptual understanding of limit among 22 undergraduate engineering students from two different sections of the same introductory differential equations course. The participants' concepts of limit (concept images and personal concept definitions) were examined using written tasks followed by one-on-one investigative interviews. The findings suggest that, at the end of their mathematics coursework, many engineering students do not consider employing a formal definition of limit to solve limit-based mathematics problems. Furthermore, the findings suggest that most participants' personal concept definitions of limit are both inoperable for solving limit problems and inconsistent with the formal definition of limit.