Penumbra means, literally, "dim light"; it is the outer filamentary region of a sunspot. However, a "penumbra" may be any area of transition around an area of great light; a lamp casts light, a shadow, and in between (the gray area) is the penumbra.
In American constitutional law, a "penumbra" refers to an area of law not clearly illumed or defined, but still important. The case Griswold v. Connecticut said that privacy in the "marital bedroom" - and privacy only in this specific area - is defined in the "penumbra" of constitutional rights, which do not specifically define a privacy right, but imply it by constitutional prohibitions against quartering and assurances of due process. Liberal jurists, such as Justice Blackmun, expanded Griswold out of step in Roe v. Wade, where it was suggested that somehow abortion is a "penumbra" right as well.