When you operate under a pretense, you're not being honest. You're pretending. If you say you're going to a party because you’re eager to see your friend, but really you're just interested in the birthday cake, you’re acting on pretense.
Pretense involves deceiving on purpose, and it's usually something you shouldn’t be proud of. Under the pretense of being a college student, you might sneak into a library restricted to enrolled students. That’s dishonest and deceitful. But pretense isn’t always bad. You might not want to hurt your mother’s feelings, so you make a pretense of loving her tuna casserole, even though you’re trying not to gag on it.