Hi,
I'd say 12 bit linearity means that the nonlinearity is not bigger than 1/(2^12) of full scale.
With ADCs and DACs you have
* differential nonlinearity (DNL)
* integral nonlinearity (INL)
If there is no additional information I assume they talk about INL .
Opamp: give us a link to the document you refer to.
Btw:
Such informations can be found:
* doing a forum search
* doing an internet search (wikipedia)
* every ADC / DAC / OPAMP manufacturer gives those informations.
Klaus