Longman Dictionary of Contemporary English
Wiktionary
n. A position for sexual intercourse in which the man and woman face each other, with the man on top.
WordNet
n. a position for sexual intercourse; a man and woman lie facing each other with the man on top; so-called because missionaries thought it the proper position for primitive peoples
Wikipedia
The missionary position or man-on-top position is generally a sex position in which a woman lies on her back and a man lies on top of her while they face each other and engage in sexual intercourse or other sexual activity. Though commonly associated with heterosexual sexual activity, the sex position is also used by gay and lesbian couples.
The missionary position may involve sexual penetration or non-penetrative sex (for example, intercrural sex), and its penile-vaginal aspect is an example of ventro-ventral (front-to-front) reproductive activity. Variations of the position allow varying degrees of vaginal tightness, clitoral stimulation, depth of penetration, participation on the part of the woman, and the likelihood and speed of orgasm.
It is commonly believed that the term missionary position arose in connection to Christian missionaries, who supposedly encouraged the sexual position to new converts in the colonial era. However, the term probably originated from Alfred Kinsey's Sexual Behavior in the Human Male through a confluence of misunderstandings and misinterpretations of historical documents. Tuscans refer to the position as the Angelic position while some Arabic-speaking groups call it the manner of serpents.
The missionary position is often preferred by couples who enjoy its romantic qualities afforded by copious skin-to-skin contact and opportunities to look into each other's eyes and kiss and caress each other. The position is also believed to be a good position for reproduction. During sexual activity, the missionary position allows the man to take charge of the rhythm and depth of pelvic thrusting. It is also possible for the woman to thrust against him by moving her hips or pushing her feet against the bed, or squeeze him closer with her arms or legs. The position is less suitable for late stages of pregnancy, or when it is desired for the woman to have greater control over rhythm and depth of penetration.