This is one of those words with a weird function confined largely to the world of law.
The ordinary current meaning of deem, according to the OED is, essentially, to consider, think or judge (in a non-judicial way).
But lawyers have a special meaning, where deeming means treating A as if it were B and not A. Creating a legal fiction, in other words (and that doesn’t mean John Grisham).
This comes up in my teaching, where the law school will occasionally stick a deemed Wednesday in the calendar, in order to make up for an actual Wednesday sacrificed for some reason like a holiday or special event.
It also comes up a lot in statutory drafting, where all kinds of things are deemed to be other things for legal purposes: the word deemed occurs 4141 times in the Income Tax Act (Canada), for example.
Oddly, the OED’s other definitions of deem, while they involve judgment of some kind, don’t quite capture the ‘treating A as if it were B’ meaning that is familiar to lawyers.
One legislative drafter says that the ordinary ‘consider’ usage should be avoided: ‘Phrases like “if he deems fit” or “as he deems necessary” are objectionable as deviations from common speech’ (GC Thornton, Legislative Drafting, 2d ed (1979), 83-4, cited in Black’s Law Dictionary).
It appears that the Dictionary would think the legislative use of deem is actually the departure from ordinary usage – but intelligent people can disagree.
This division of opinion may cease to matter. Ruth Sullivan suggests that modern legal drafters have already ditched deem in the ‘consider’ sense and are starting to say is considered or just is when they wish to create legal fictions (Sullivan on the Construction of Statutes, 6th ed (Markham, Ont: LexisNexis Canada, 2014), at §4.105).