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Uncommitted Logic Array.

It was a semi-custom chip type, where the silicon layer was standardised but the metal layers varied by customer.

https://en.wikipedia.org/wiki/Gate_array

Just want to add that they're very common on 1980s computers in Britain. Sinclairs and Amstrads have them. And that they often fail and are very hard to replace. Essentially you end up taking one from another machine, so the stock of working machines declines over time.

Would it be possible to recreate the ULA logic in a modern FPGA? And if so, to use it as a direct replacement in an original Sinclair/Amstrad?

Not only possible, but doable. Not only doable, but already done :)

https://www.sellmyretro.com/offer/details/vla81-%3A-a-replac...

https://www.sellmyretro.com/offer/details/vla82-%3A-a-replac...

Yes, but at that point, couldn't you put the rest of the device in the FPGA too?

Yup! https://github.com/MiSTer-devel/ZX81_MiSTer

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