Magma
Magma is molten or partially molten rock beneath Earth's surface, containing dissolved gases and suspended crystals, that becomes lava when it erupts.
Definition
Magma is melted rock still underground, a hot mixture of liquid silicate melt, suspended crystals, and dissolved gases such as water vapor, carbon dioxide, and sulfur dioxide that becomes lava once it erupts. It forms when mantle or crustal rock partially melts from decreased pressure, added water, or rising temperature, and being less dense than the surrounding rock, it rises and collects in magma chambers, ranging in composition from runny basaltic to thick rhyolitic. Viscosity spans many orders of magnitude, from about $10$ to $100\ \text{Pa}\cdot\text{s}$ for basalt to over $10^8$ for rhyolite, controlled by silica polymerization, temperature, crystal content, and dissolved water, and magma evolves further by fractional crystallization along Bowen's series, by assimilating surrounding rock, and by mixing with other magmas.
Example
A large pocket of magma under Yellowstone heats the water that feeds the park's geysers and hot springs, with basaltic magma near $1{,}200^\circ C$ feeding Hawaiian volcanoes and cooler rhyolitic magma near $800^\circ C$ sitting beneath Yellowstone, and granite is simply magma that cooled slowly underground without ever erupting. Seismic imaging of the Yellowstone reservoir, estimated at about $10{,}000\ \text{km}^3$ of rhyolite, shows it is mostly crystal mush with only about $5$ to $15\%$ melt, far below the roughly $50\%$ needed for it to erupt, while kimberlite magmas rise from depths over $150\ \text{km}$ fast enough to carry diamonds to the surface intact.
Key Insight
Magma and lava are the same material, distinguished only by whether it is below or above the ground, and most magma never erupts at all, instead cooling slowly underground into granite and other intrusive rock, which is why granite is so common in the cores of old mountain ranges. The older picture of a single liquid magma chamber is also being replaced by a mush model, in which most magma bodies are largely solid, crystal-rich frameworks with melt only in the pore spaces, needing a fresh pulse of heat to remobilize before they can erupt.