Two real numbers or two algebraic expressions related by the symbol '<', V, '<' or '>' form an inequality.
For example: 3 < 5, 7 > 5 are numerical inequalities x < 5, y > 2, x > 3 are literal inequalities. ax + b < 0, ax + b > 0 are strick inequalities in one variable.
ax + b < 0, ax + b > 0 are slack inequalities in one variable.
ax + by < c, ax + by > c are strick inequalities in two variables.
ax + by < c, ax + by > c are slack inequalities in two variables.
$$ax^2 + by < c, ax^2 + by > c$$ are quadratic inequalities in one variable.