a material property
A property is a measurable trait that tells you how a material responds to some outside influence - a force, an electric field, heat, light, a chemical environment - in a way that does not depend on how much of the material you have or its shape. Density, stiffness, melting point, electrical resistance, and color are all properties. They let you compare a lump of copper with a lump of glass fairly, on equal footing.
Engineers group properties by the kind of stimulus. Mechanical (response to force: stiffness, strength, hardness, toughness), electrical (resistivity, dielectric constant), thermal (heat capacity, thermal expansion, conductivity), magnetic, optical (color, transparency, refractive index), and deteriorative or chemical (corrosion and oxidation resistance). A property is measured by a standardized test - pulling a sample in a tensile test to get its strength, say, or pressing a hard tip into it to get its hardness. Numbers come with units: Young's modulus in GPa, resistivity in ohm-meter.
Be honest about two things. Properties are not all independent, and single numbers hide scatter. Strength, stiffness, and toughness are different properties - a ceramic can be very stiff and strong yet shatter (low toughness), while annealed copper is soft yet very tough. And real measurements scatter from sample to sample, so engineers apply a safety factor rather than trusting one exact value.
Steel and aluminum both feel 'strong', but steel's Young's modulus is about 200 GPa versus aluminum's ~70 GPa - steel is roughly three times stiffer. That comparison holds regardless of the part's size or shape.
Properties let you compare materials fairly, apart from size and shape.
Do not confuse strength (resistance to permanent deformation or breaking) with stiffness (resistance to elastic bending). A fishing rod is strong but not stiff; a glass rod is stiff but not strong. They are separate properties set by different things.