What term refers to the business or professional activity that provides the principal source of income?

The term "Principal Business or Profession" accurately refers to the primary business or professional activity that serves as the main source of income for an individual or entity. This terminology is often used in tax-related contexts to identify the main economic pursuits that generate earnings, distinguishing them from secondary sources of income.

In tax filings, understanding one's principal business or profession is crucial for reporting income accurately and determining allowable deductions related to that business. It represents the core operation of a taxpayer’s activities from which they derive the majority of their earnings.

The other options, while potentially relevant in a general discussion about income sources, do not specifically align with the terminology used in tax contexts. For instance, "Primary Business Activity" is somewhat ambiguous and may not clearly define the connection to income generation. "Main Source of Revenue" implies an informal usage and can relate to any revenue-generating aspect, not strictly defining a business activity. "Major Income Stream" also lacks the specificity of pinpointing a recognized business or profession, making it less accurate. Thus, "Principal Business or Profession" is the most precise term in this context.