According to the U.S. Census Bureau FAQ, "median income is the amount which divides the income distribution into two equal groups, half having income above that amount, and half having income below that amount. Mean income (average) is the amount obtained by dividing the total aggregate income of a group by the number of units in that group. The means and medians for households and families are based on all households and families. Means and medians for people are based on people 15 years old and over with income."
U.S. Census Bureau, Small Area Income and Poverty Estimates (SAIPE). Estimates for Mississippi Counties. Median household income, in dollars, [Year].
Available at: https://www.census.gov/data/data-tools/saipe-interactive.html
Updated December 2017.