'Obsolete' is defined as

No longer produced or used; out of date.

and 'defunct' is defined as

No longer existing or functioning.

The meanings sound close to each other though the latter should be more formal. Are there situations in which native English speakers use them interchangeably?

An example from OALD:

The LP was made defunct by the arrival of the CD.

  • 2
    No, they aren't the same. – marcellothearcane Sep 2 '19 at 11:54
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    Some obsolete equipment still performs perfectly well - even though the producing company is now defunct. – Davo Sep 2 '19 at 11:59
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    A brand new automobile can be defunct if the engine throws a rod. – Hot Licks Sep 2 '19 at 12:08
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    No, they convey to distinct ideas, for example, "Your air conditioner's coil is defunct. I'd order you a new one, but it's also obsolete. As such, you will have to replace the entire air conditioner unit." – Benjamin Harman Sep 2 '19 at 13:38

Defunct refers to the company making a product and obsolete refers to the product itself.

For example, consider Netscape Navigator:

Netscape Navigator was a proprietary web browser, and the original browser of the Netscape line, from versions 1 to 4.08, and 9.x. It was the flagship product of the Netscape Communications Corp and was the dominant web browser in terms of usage share in the 1990s, but by 2002 its use had almost disappeared. source

Netscape Communications Corp is defunct as the company is no longer in business. It was bought by AOL. In contrast, its Netscape Navigator is obsolete. This browser cannot handle many modern HTML conventions.

The OP cited an example of a defunct rail line. Lexico.com

the now defunct Somerset & Dorset railway line

This means that the line is no longer being used (and perhaps stopped carrying passengers only a week ago). It is not functioning at the moment, but could be in the future. That defunct railway line could be dusted off and recommissioned next week. While not broken, it is not being used in its original capacity.

If the manufacturer of the train were out of business, then it would be defunct, as well.

The engines on that rail line could be obsolete steam locomotives. But you would not call the engines defunct.

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The answer is in the definitions you have given. 'Obsolete' is No longer produced or used; out of date. - this does not mean it is non-functioning. 'Defunct' means it is broken - No longer existing or functioning.

Here's an example:

  • A perfectly working iPhone 5 could be called obsolete - it is out of date, since newer models exist.
  • An iPhone XR, the latest at time of writing (hah), could smash as soon as you open the box. It is now defunct, but not obsolete. It would become both defunct and obsolete when you replace it.
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  • 1
    You might want to correct your spelling of obsolete. And I don't see how you can call an iPhone XR defunct as Apple is producing it. – rajah9 Sep 2 '19 at 12:27
  • @rajah thanks for the edit. As stated, if you smashed it immediately, it would be defunct. A pristine XR would be neither obsolete nor defunct. – marcellothearcane Sep 2 '19 at 12:42
  • What about "The LP was made defunct by the arrival of the CD"? (source: OALD) – apadana Sep 2 '19 at 13:54
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    @apadana Not really true, for quite a long time both LPs and the equipment to play them were being produced in parallel with CDs and CD players. In fact you could buy equipment with all three of a conventional disc deck a tape deck and a CD deck in the same enclosure. The CD eventually made LPs obsolete but they never became defunct. – BoldBen Sep 2 '19 at 14:00
  • This answer incorrectly equates broken with not functioning. The OP reference cites an example of a defunct railway line. This means the railway line is not functioning in its original capacity. It does not mean that the railway line is broken. Conversely, a working railway line is not made defunct when a saboteur blows up its tracks. – rajah9 Sep 2 '19 at 14:16

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