Ejaculation is the ejecting of semen
Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...

 (usually carrying sperm) from the male reproductory tract
Male reproductive system (human)
The human male reproductive system consists of a number of sex organs that are a part of the human reproductive process...

, and is usually accompanied by orgasm
Orgasm is the peak of the plateau phase of the sexual response cycle, characterized by an intense sensation of pleasure...

. It is usually the final stage and natural objective of male sexual stimulation
Sexual stimulation
Sexual stimulation is any stimulus that leads to, enhances and maintains sexual arousal, and may lead to ejaculation and/or orgasm...

, and an essential component of natural conception
Fertilisation is the fusion of gametes to produce a new organism. In animals, the process involves the fusion of an ovum with a sperm, which eventually leads to the development of an embryo...

. In rare cases ejaculation occurs because of prostatic disease. Ejaculation may also occur spontaneously during sleep
Sleep is a naturally recurring state characterized by reduced or absent consciousness, relatively suspended sensory activity, and inactivity of nearly all voluntary muscles. It is distinguished from quiet wakefulness by a decreased ability to react to stimuli, and is more easily reversible than...

 (a nocturnal emission
Nocturnal emission
A nocturnal emission involves either ejaculation during sleep for a male, or lubrication of the vagina for a female. It is also called a wet dream, and is sometimes considered a type of spontaneous orgasm....

 or "wet dream"). Anejaculation
Anejaculation is the pathological inability to ejaculate in males, with or without orgasm.It can depend on one or more of several causes, including ':*Sexual inhibition*Pharmacological inhibition...

is the condition of being unable to ejaculate.
A usual precursor to ejaculation is the sexual arousal
Sexual arousal
Sexual arousal, or sexual excitement, is the arousal of sexual desire, during or in anticipation of sexual activity. Things that precipitate human sexual arousal are called erotic stimuli, or colloquially known as turn-ons. There are many potential stimuli, both physical or mental, which can cause...

 of the male, leading to the erection
Penile erection is a physiological phenomenon where the penis becomes enlarged and firm. Penile erection is the result of a complex interaction of psychological, neural, vascular and endocrine factors, and is usually, though not exclusively, associated with sexual arousal...

 of the penis
The penis is a biological feature of male animals including both vertebrates and invertebrates...

, though not every arousal nor erection leads to ejaculation.