Logograms of the heptapods. To wit:
A logogram’s weight carries meaning, too: A thicker swirl of ink can indicate a sense of urgency; a thinner one suggests a quiet tone. A small hook attached to one symbol makes it a question. The system allows each logogram to express a bundle of ideas without adhering to any traditional rules of syntax or sequence.
MORE: How Arrival’s Designers Crafted a Mesmerising Alien Alphabet (Margaret Rhodes, Wired) (minor spoilers)