Generally refers to the combined net value of all investment products one owns. Some common investment products include stock, stock options, bonds, cash, real estate, intellectual property and loans, among others.
An individual can own assets like stocks, cash in money market accounts, bonds. That individual also owns liabilities such as auto loans, mortgage on his home, and other debt such as credit card bills etc.
equity 14c., from equite (13c.), "equality, conformity, symmetry, fairness," from aequus "even, just, equal" (see equal). As the name of a system of law, 1590s, from Roman naturalis aequitas, the general principles of justice which corrected or supplemented the legal codes. 
equal (adj.) 14c, "uniform, identical, equal," from aequus "level, even, just," of unknown origin. Parallel formation egal was in use late 14c.-17c. The noun is recorded from 1570s. The verb is from 1580s, "compare, liken," also "match, rival."