Sanskrit मैत्रेय, "benevolent, loving, kind" from मित्र mitra "friend."
Like the word Buddha, the word maitreya is a title, not a personal name. Anyone who enters the path of the bodhisattva and fully develops bodhichitta becomes a maitreya buddha, which literally means "one who is awakened in compassion." Not all buddhas follow the bodhisattva path (the direct path); most buddhas are buddhas pratyekas (nirvanis of the spiral path). As in esoteric Christianity we call Christ anyone who incarnates the Cosmic Christ and thereby becomes an expression of that force, likewise, in esoteric Buddhism we call Maitreya any buddha who renounces nirvana for the sake of those who are suffering in ignorance.
"A permanent buddha or buddha of contemplation is a buddha who has already Christified themselves, a buddha that has already received the inner Christ (bodhichitta / Chenresig / Avalokitesvara) within their internal nature. This type of Buddha is a Buddha Maitreya, since it is a buddha who incarnated the inner Christ (this is how the term “maitreya” should be understood). So, buddha maitreya is not a person: buddha maitreya is a title, a degree, which indicates any given buddha who already achieved Christification [ie. the full development of bodhichitta]." - Samael Aun Weor, The Major Mysteries
In Mahayana Buddhism, there are several buddhas called Maitreya, and people assume they are all one person; in reality, there are many buddhas called Maitreya, since the word is not a personal name.
There are many scriptures said to be given by Maitreya; this is true in the sense that they were given by the compassion of the awakened masters. However, it does not mean that all those scriptures were given by one person.
The word Maitreya is also used to refer to the Buddha of the coming era, and is the only Buddhist figure represented seated in a chair, Western style.
A Russian traveller, Nicholas Roerich, recorded the following prediction made by a Tibetan lama:
“First will begin an unprecedented war of all nations. Afterward, brother shall rise against brother. Oceans of blood shall flow. They shall forget the meaning of the word Teacher (buddha, guru). But then shall the Teachers appear and in all corners of the world shall be heard the true teaching. To this word of truth shall the people be drawn, but those who are filled with darkness and ignorance shall set obstacles. As a diamond, glows the light on the tower of the Lord of Shambhala. One stone in his ring is worth more than all the world’s treasures. Even those who by accident help the Teachings of Shambhala will receive in return a hundredfold. Already many warriors of the teaching or truth are reincarnated. Only a few years shall elapse before everyone shall hear the mighty steps of the Lord of the New Era. And one can already perceive unusual manifestations and encounter unusual people. Already they open the gates of knowledge and ripened fruits are falling from the trees. The banner of Shambhala shall encircle the central lands of the Blessed One. Those who accept him shall rejoice. And those who deny him shall tremble. The denier shall be given over to justice and shall be forgotten. And the warriors shall march under the banner of Maitreya.”