Logic block (CLB, LAB, slice, ALM)
Beginner
A small group of lookup tables and one-bit memories that sits in the FPGA’s grid.
Novice
The repeated unit of an FPGA’s general-purpose logic: several LUTs, flip-flops, carry logic and local wiring. AMD calls it a CLB made of slices, Altera a LAB made of ALMs, Lattice a logic tile or PFU, Microchip a logic cluster.
Expert
A cluster of N basic logic elements (K-LUT + flip-flop) with a local crossbar and shared control signals. Cluster size trades fast local connections against crossbar area; inputs scale near K/2·(N+1).
Explained in Lookup tables and the FPGA fabric (Architectures).
See also: LUT (lookup table), Carry chain, Flip-flop (register).