The interoperability among a variety of systems, in or across manufacturing enterprises, has been widely accepted as one of the important factors that affect the efficiency of production. Many research works, related to the interoperability at different levels, have been carried out to tackle the information exchange, transformation, discovery and reuse. One of the main challenges in these research works is to overcome the semantic heterogeneity in the exchanged information between enterprise applications along the life cycle of a product. As a possible solution to support the semantic interoperability issue, semantic annotations have gained more and more attention. This paper identifies several existing drawbacks and proposes a formal semantic annotation approach to support the semantic enrichment of models in a product lifecycle management context.