Shaila Catherine, the principal teacher of Insight Meditation South Bay, is one of the foremost teachers of samadhi, or concentration meditation in the West. She translates samadhi as concentration, the most common translation. Her website includes a good introduction to the meaning of concentration and ways in which to practice it and develop it. You can find her introduction here: Cultivating Concentration
Ajahn Sucitto describes the connection between joy and samadhi in an article in which he makes clear that “through this process of bringing happiness into body and mind, the mind is naturally concentrated. That is samādhi.” So, concentration is fostered by joy, not the forceful exertion that many expect it to be. You can find his article here: Samadhi is Pure Enjoyment