Is the administration of therapeutic agents before a main treatment.
One example is neoadjuvant hormone therapy prior to radical radiotherapy for adenocarcinoma of the prostate.
Neoadjuvant therapy aims to reduce the size or extent of the cancer before using radical treatment intervention, thus making procedures easier and more likely to succeed, and reducing the consequences of a more extensive treatment technique that would be required if the tumor wasn't reduced in size or extent.

Another related concept is that neoadjuvant therapy acts on micrometastatic disease. The downstaging is then a surrogate marker of efficacy on undetected dissemination, resulting in improved longtime survival compared to the surgery-alone strategy.

This systemic therapy (chemotherapy, immunotherapy or hormone therapy) or radiation therapy is commonly used in cancers that are locally advanced — where clinicians plan an operation at a later stage. The use of such therapy can effectively reduce the difficulty and morbidity of more extensive procedures.

Adapted from Wikipedia, the free encyclopedia. Internet. Accessed on January 18, 2016.