Can't speak for other legal systems, but under UK law (and other common-law systems are often similar), deception can be implied by conduct. It is certainly NOT necessary to explicitly "lie to the flight attendant." The airline gives you a boarding pass - and invites you to board. By taking up a seat and accepting services there is a deception here as to your status - you imply that you are someone entitled to sit in that seat in that cabin class. Indeed if you made an innocent mistake in taking up the wrong seat and later realised your mistake you would be guilty of a deception just by remaining silent and continuing to enjoy the service.

The main statute is the Theft Act 1968 as updated by 1978 and 1996 acts.
s1 Theft Act 1978 as amended by the Theft (Amendment) Act 1996 gives:

(1) A person who by any deception dishonestly obtains services from another shall be guilty of an offence.

(2) It is an obtaining of services where the other is induced to confer a benefit by doing some act, or causing or permitting some act to be done, on the understanding that the benefit has been or will be paid for.

No doubt things have diverged a little in the US, but note that in xanthuos's quote "concealment, false declaration, or artifice", there is the word "or". On this basis there does not have to be a false declaration - just an "artiface" would be enough.

Frankly if considering criminal law, case law is a better guide to "what counts" as a deception than dictionary definitions, especially if you have to follow through several definitions.

In any case, "there are many ways to skin a cat". In the UK a simple s1 Theft Act 1968 charge could be simpler once you've started "appropriating property":

s1: "A person is guilty of theft if he dishonestly appropriates property belonging to another with the intention of permanently depriving the other of it.."

The various terms are then defined in subsequent sections and are clear enough. A glass of orange juice would be enough - even if you could have got the same juice in economy.

The current legal test for dishonesty is doing something that:
1) is dishonest by the standards of ordinary and reasonable people and
2) knowing that the act was dishonest according to those standards.

So its no defence to say "I didn't think what I did was wrong". But you would get away with it if you could convince a jury that you thought normal people would have thought its OK. This aspect of the test is a balance of probabilities thing, not a beyond reasonable doubt thing.

Can't think anyone would try and prosecute it though.

As for a civil action, the airline could look at trespass or the provisions of the contract they have with pax. I should think damages from trespass would be pretty small but they might have entitled themselves to remove your airmiles, deny you boarding in the future etc.

Of course if there is any consent, then nothing is amiss.

(not a lawyer, yet, give me a job)