A form of sofa with a backrest at one end only.
'Chaise lounge' appears to be a very early example of an eggcorn, dating from long before eggcorns were given a name.
[Note: eggcorns are words or phrases which have been coined mistakenly, often due to an incorrect guess as to how a word is spelled, but one which makes some kind of sense; for example, Old-Timer's Disease for Alzheimer's Disease and daring-do for derring-do.]
The misspelling 'chaise lounge' is the object of much derision outside the USA.
'Chaise lounge', also sometimes spelled 'chase lounge', began life as such a linguistic mistake and has survived because it does make intuitive sense. The piece of furniture in question is properly called a 'chaise longue'. The name is of French origin, of course, and has been known there since the 18th century, translating into English simply as 'long chair' - which is just what it is. The understandability of the misspelling of 'longue' as 'lounge' is that lounging is what one does on these sofas and the supposed translation of 'lounging chair' makes perfect sense.
The spelling and pronunciation as 'chaise lounge' is largely limited to America. It is so well-established there that it is far too late to turn back the clock - only the most foolhardy of visitors to the USA would attempt to flag it as a mistake. The confusion for those outside America is added to by the fact that those items of garden or poolside furniture which are known in most other English-speaking countries as a 'sun-loungers', are also often called 'chaise lounges' in the USA.
Those English-speakers from the Mother Country who look down on this as a typical mangling of the language by those uncultured Yanks might be dismayed to find that the earliest known citation of the 'chaise lounge' spelling comes from no less a bastion of 'proper' English than The Times newspaper. The January 16th 1807 edition included an advert for An Assemblage of truly elegant furniture, fitted up in the most modern style, and this includes the offer of "sofas, chaise lounge, loo tables" etc. The earliest citation that I can find from America, and which appears to refer to a 'sun-lounger', is in the 4th January 1875 edition of The Newport Daily News:
"A real rattan chaise lounge, such as is made at Singapore."
See other phrases that were coined in the USA.