In order to synthesize EFL vocabulary acquisition research, a concept of acquisition unit is needed that emcompasses various lexical phenomena and has sufficient explanatory power. It is argued in this paper that such a concept must meet the requirements of inclusiveness and applicability. Whereas the “word” concept is deeprooted in vocabulary acquisition research and may exert unfavorable impact on vocabulary teaching and learning as well as testing, the “lexical unit” concept implies a neglect of the teaching and learning of the function words, which undermines the vital importance of the function words in L2 use. These two concepts fall short of inclusiveness and applicability. In this paper I propose a concept of “lexicunion” as the acquisition unit of EFL vocabulary. The “lexicunion” concept satisfies both inclusiveness and applicability, and at the same time considers the burden involved in both learning and using L2 vocabulary, thus better reflecting the complexity of the process of L2 vocabulary acquisition. This concept may be useful in discussing the interaction between EFL learners’ linguistic competence and their vocabulary knowledge, and may eventually contribute to the synthesis of L2 vocabulary acquisition research that is based on different acquisition units.