It is the combinations of quantities of X and Y that a consumer can purchase with certain amount of income or earning. 

It is the minimum amount of the goods that a person can afford using his or her income or earnings.

It is the gap between the desired goods and the afforded goods for a person who works for a stable salary.

It is the unexpected increase in budget line of a person due to some unexpected expenses such as car insurance due to an accident.

It is the decrease in a persons budget caused by gradual  increase in inflation and global stocks.