Malignant transformation is one of the steps during carcinogenesis (cancer generation), which is typically divided into initiation, promotion, malignant transformation and progression. It is the change from a pre-neoplastic cell into one that expresses malignant phenotype: for example, the proliferation is uncontrolled and the apoptosis control has failed. It requires genetic modification of the parent cells.
http://www.ncbi.nlm.nih.gov/books/NBK13982/