A burden is a heavy responsibility or obligation that someone has to carry. The term 'a burden' can be broken down into two words: 'ONUS' and 'LOAD'. ONUS is defined as an obligation, duty, or burden, while LOAD is defined as a large amount of something that has to be carried or dealt with. Together, these two words form the phrase 'a burden', which is a heavy responsibility or obligation that someone has to carry.