Mathematical language (or the content-specific words in mathematics) has repeatedly shown to be related to mathematical abilities in preschool and in the early grades of primary education. Research in this field has predominantly focused on young children's quantitative and spatial language. At the same time, recent research has discovered that children in the early years of primary education already possess advanced mathematical skills such as reasoning about patterns, proportions and probabilities. In this study we developed an advanced mathematical language task to measure children's understanding of mathematical language proper of the domains of patterning, proportionality, and probability, specifically aimed at first, second, and third graders (ages 5-8). To develop a valid test instrument, our task was substantiated by a review of relevant literature and previously conducted studies. Further, the task was optimized based on the feedback received from expert professionals and a pilot study with 36 children. After administering the advanced mathematical language task to 236 first, second, and third graders, the results suggest that the task provides reliable scores in these grades. The internal consistency is excellent ([alpha] = 0.993). In addition, the test-retest reliability was checked in 49 children (r = 0.704; p < 0.001), and shows that the test also produces stable results over time.