LD stands for Lethal Dose. LD50 is the amount of a toxic substance which, when given all at once, causes the death of 50% of a group of test organisms. 

TD stands for Toxic Dose. TD50 is the median toxic dose of a substance at which toxicity occurs in 50% of the cases.

A substance that is toxic is not necessarily lethal. Some toxic substances may cause some degree of damage to an organism but they do not cause its death. The TD50 of a substance is usually lower than its LD50. In most cases, the LD50 is considered as the upper limit for the TD50.