Question

which interval does the inequality (x)<3 define

Answer 1

An open interval from -infinity to 3 not including 3. You could write it as (-infinity,3)

Answer 2

It is all the numbers that are less than 3. 3 is not included. or you could say all the numbers where 3 is the largest number. You can read inequalities in either direction
x<3 or 3>x