Lead time bias refers to the phenomenon where early diagnosis of a disease falsely makes it look like people are surviving longer. This occurs most frequently in the context of screening.
For example, a man with metastatic lung cancer dies at age 70. His cancer was discovered 1 year ago, when he was 69. Therefore, it appears as if he lived for 1 year with the cancer. However, imagine that instead his cancer was discovered on a screening CT scan when he was 65 years old. If he still dies at the age of 70, it now looks like he survived for 5 years with the diagnosis of cancer (the 5 year survival rate is much better), but in fact there was no real change in his survival.
Gates TJ. Screening for cancer: evaluating the evidence. American family physician. 2001; 63(3):513-22. [pubmed]