Vaanprasthi [वाणप्रस्थी ]: A person, who has mostly fulfilled duties towards his immediate family, which consists of self+spouse+own children+parents.
Yagna [यज्ञ ]: An action/ activity performed selflessly by a group of people for the larger benefit of the group and beyond.
Brahma [ ब्रह्म ]: The utlimate from which whole universe as emerged, which engulfs every thing, which manifests as every thing that we know, which reside in every thing. It is inside and outside every thing. Some also call it Isha, Parmatma, Om, etc.
Brahman [ब्राह्मण]: Someone [human] whose man [मन] is in Brahma, who knows/ understands Brahma and leads a life with that knowledge.
Brahmnatva [ब्राह्मणत्व]: Attributes of a Brahman.
Brahmchari: Someone [human] who spreads the knowledge of Brahma. [Chari -- चारी : ले जाने वाला ].