Pull-up resistor
A resistor from a signal to the positive rail that defines the signal's idle state as HIGH. Anything that can only pull the line low — an open-collector output, a button to ground, an I²C device — needs one to make the high state exist.
In practice
Value is a speed/power trade: strong (1–4.7 k) for fast edges on capacitive buses, weak (10–100 k) to save current. On I²C the pull-up and the bus capacitance literally set the rise time — too weak and the bus drops bits at length.