not usual; often refers to the appearance of precancerous or cancerous cells.
a term indicating suspicious cells that could be cancer, but not confirmed as such. For instance, lymphomas (cancers of the lymph nodes) are notoriously difficult to diagnose. Some lymph node biopsies are very disturbing but do not quite fulfill the criteria for cancer.
The simple, straightforward definition would be "unusual," but "atypical" means much more than that. In a diagnosis, the use of the term atypical is a vague warning to the physician that the pathologist is worried about something, but not worried enough to say that the patient has cancer. For instance, lymphomas (cancers of the lymph nodes) are notoriously difficult to diagnose. Some lymph node biopsies are very disturbing but do not quite fulfil the criteria for cancer. Such a case may be diagnosed as "atypical lymphoid HYPERPLASIA." Other important atypical hyperplasias are those of the breast (atypical ductal hyperplasia and atypical lobular hyperplasia) and the lining of the uterus (atypical endometrial hyperplasia). Both of these conditions are thought to be precursor warning signs that the patient is at high risk of developing cancer of the respective organ (breast and uterus).
Literally, "not typical." Exhibits unusual characteristics. For example, atypical hyperplasia is a dangerous increase in the number of breast cells; a sign that breast cancer may develop.