Hybrid (h) parameters
Hybrid parameters are a tidy four-number summary that describes how a transistor (or any two-port black box) behaves for small signals, without you needing to know its internal physics. They were the engineer's everyday language for the BJT for decades: read four numbers off a datasheet and you can predict the amplifier's gain, input resistance and output resistance.
The four are h_ie (input resistance), h_re (a usually-tiny reverse voltage feedback), h_fe (forward current gain, the famous β), and h_oe (output conductance). They are called 'hybrid' because they mix units — some are ohms, some are siemens, some are dimensionless ratios — chosen so each is easy to measure: short or open the right port and read it directly.
h-parameters and the small-signal hybrid-π model describe the same transistor; h-parameters are easier to measure on a curve tracer, while the hybrid-π (with g_m and r_π) is more intuitive for hand analysis — and they convert directly into each other.