What does CISC stand for and what characterizes it?

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CISC = Complex Instruction Set Computer: many instructions, variable length, rich addressing modes, and complex operations broken into microcode internally.
The CISC idea is "let one instruction do a lot." x86 is the archetype. Its instructions vary from 1 to 15 bytes, many can take a memory operand directly, and a single mnemonic can combine an address calculation, a memory read, and an arithmetic op. Complex instructions are decoded inside the chip into simpler internal micro-ops.
A classic CISC one-liner — read from memory, scale an index, and add, all in one instruction:
add (%rax,%rbx,4), %rcx   # %rcx += memory[%rax + %rbx*4]

Advantage: compact code — historically valuable when memory was expensive — and fewer instructions to express an operation.
Disadvantage: the decoder is complicated, instruction timing is uneven, and variable lengths make pipelining harder. This is exactly the cost RISC set out to avoid.
Go deeper:

CISC (Wikipedia) — CISC's defining trait of multi-operation instructions.