Scalar
Definition
A quantity described by a single number with no direction — like temperature (25°C), mass (5 kg), or the time on a clock.
Technical definition
A physical quantity fully specified by its magnitude (and unit), with no directional component. Scalars combine by ordinary arithmetic; examples include mass, temperature, energy, speed, and time.
Example
Two 5 kg bags of flour make 10 kg total — masses add as scalars. But two 5-newton pushes in opposite directions cancel out, because force is a vector and direction matters.
Common confusion
Speed versus velocity is the classic trap: speed is a scalar (how fast), velocity is a vector (how fast plus which way). A car turning a corner at steady speed has constant speed but changing velocity — the distinction matters for acceleration.