This work is written to enable researchers and students to better understand the specialised language of Teilhard's transdisciplinary approach, which he found himself compelled to develop as a means of expressing that extraordinary vision of a universe in process of convergence towards a cosmic centre of unity he identifies with the Cosmic Christ. All quotations in the Lexicon have been sourced from the French originals. It includes a full bibliography of Teilhard's works in French and English. The year 2005 marks the 50th anniversary of Teilhard's death. Several conferences are planned to mark this anniversary.