Aspect-oriented software development (AOSD) has great potential in reducing software complexity. Aspects have been defined in the implementation phase of software development, but lack clear understanding in the early phases of software development. Without this early focus on aspects, the benefits of aspect-oriented programming are lost. This paper proposes a definition of what an aspect is in the requirements phase of software development that focuses on both functional and non-functional requirements. In addition, this paper presents a methodology, the SLAI (Structured Lexicon for Aspectual Identification) Methodology, for the systematic identification of aspects at this stage. This methodology examines all the vocabulary used to define the requirements of the system to ensure that all terms are reused as much as possible, eliminating similar terms for the same concepts. The SLAI was used in a case study where requirements were systematically analyzed and aspects were identified from both functional and non-functional requirements.