Young Brunette Girl Wearing A Necklace At The Window In The Morning

A beam over the top of a window is known as the lintel or transom, while a beam under the bottom of a window is known as the sill plate or window sill.
In the US, the NFRC Window Label lists the following terms:
• Thermal transmittance (U-factor). Best values are around U-0.15 (equal to 0.8 W/m2/K).
• Solar heat gain coefficient (SHGC) (ratio of solar heat (infrared) passing through the glass to incident solar heat)