This paper contributes a definition of "mobile literacy." This is worthwhile because although mobile, internet-enabled devices are increasingly prevalent in many people's lives, mobile literacy appears to be under-theorised and lacking definition. After giving an overview of the scale and nature of mobile device use, the paper develops the definition through building on an existing body of work which seeks to define literacies, digital literacies and mobile learning. The definition takes account of the mobility of technology, of learners, and of learning. A systematic multimodal analysis of a complex undergraduate text, in the form of a conference-style poster, is then undertaken in order to exemplify the definition offered. The analysis attends to both the semiotic resources exploited by the text's author and the wider context the text is created within. Interview data complements that constructed through analysis of the text itself. Combining these two data sources reveals the ideational, interpersonal and textual/organisational meanings communicated by the text, and how mobility is a contextual factor which is fundamental to the literacy practices employed by the author to convey these meanings.