The word "pure" means that something is free from contamination. In the Biblical sense, it would seem that she became impure after submitting to Alec's seductive ways for a couple of months in her naive youth. Clearly, she had been taken advantage of and couldn't comprehend the full manitude of her actions at that critical time in her life. Some may say that since it isn't her fault, that she was a victim, then her purity remains strong. Others may say that she rejected her old life, repented per se, and turned her life around, so that makes her pure. As far as the author is concerned, he certainly must have thought her pure enough to include in the title of the story and to make a point to those in his generation. It would seem that Hardy used Tess's story as a way to rebel against the ideal of the time which forbade women from sexual freedom as men like Angel enjoyed. Hence, the question of her purity remains to be answered by the individual if the author's view is not considered.