Ashok Kumar Behuria replies: There is an overlap between the two. However, they are different. Indian subcontinent is a subset of South Asia, as much as both are subsets of the Asian continent.
As a geographical expression, Indian subcontinent encompassed the British colonial administrative unit called India, which comprised of the present day states of India, Pakistan and Bangladesh. Till 1936, Burma (now Myanmar) was also part of British India, but was not considered part of the subcontinent.