Transvestic fetishism is a sexual fetish for the clothing of the opposite gender. It is one of a number of cross-dressing behaviours and is primarily a psychiatric term. (Attraction to the clothing of the same gender is called homeovestism).
Transvestic fetishism, fetishistic transvestism and sometimes transvestism are also often used to describe any sexual behaviour or arousal that is in any way triggered by the clothes of the other gender. Especially the latter is problematic, because transvestism and cross-dressing are neither a sexual fetish, nor do they necessarily have anything to do with sexual behaviour or arousal.
Also, not every sexual behaviour where clothes of the opposite gender are involved is transvestic fetishism, they are also often used in sexual roleplay without being a fetish. Also, many transgendered people, mostly transwomen, also cross-dress before coming out in sexual contexts to relieve their cross-gender feelings. This behaviour is likewise not considered transvestic fetishism, as it is not cross-dressing for sexual pleasure, rather it is simply their self-gender expression.
Some male transvestic fetishists collect women's clothing, e.g. nightgowns, babydolls, slips, and other types of nightwear, lingerie stockings and pantyhose, items of a distinct feminine look and feel. They may dress in these feminine garments and take photographs of themselves while living out their secret fantasies. Many men love the feeling of wearing silk or nylon and adore the silky fabric of women's nightwear, lingerie and nylons.
Most transvestic fetishists are said to be heterosexual men, although there are no studies that accurately represent either their sexual orientation or gender, and most information on this is based on anecdotal evidence or informal surveys.
A small number of people with transvestic fetishism, as the years pass, want to dress and live permanently as women, and desire surgical or hormonal sex reassignment. In such cases the diagnosis should be changed to transsexualism. 
As a psychiatric condition
There are two key criteria before a psychiatric diagnosis of "transvestic fetishism" is made:
1. Recurrent, intense sexually arousing fantasies, urges, or behaviour, involving cross-dressing.
2. This causes clinically significant distress or impairment, whether socially, at work, or elsewhere.
Thus, transvestic fetishism is not considered a mental illness unless it causes significant problems for the person concerned, however many people who are diagnosed with transvestic fetishism are against their fetish being described as an illness. 
Transvestic fetishists may begin wearing their opposite sex's clothing as young children, using the clothes of a sibling, parent, or friend. Some parents have said they allowed their children to cross-dress and in many cases the child stopped when they became older. It appears that when boys are forbidden to cross-dress, they will try to stop, only to resume doing it later. The same pattern often continues into adulthood, where there may be confrontations with a spouse. Married transvestic fetishists will experience considerable anxiety and guilt if their spouse objects to their behaviour. Some have periodically disposed of all their clothing, a practice called "purging", only to start another collection later.
* Stratton, Jon (1996). The Desirable Body: Cultural Fetishism and the Erotics of Consumption. Manchester University Press. ISBN 0-7190-4701-3
* Tyler, Carole-Anne (2003). Female Impersonation. Routledge. ISBN 0-415-91687-9
1. ^ Genderpsychology.org.
2. ^ Gender Identity Disorder Reform
3. ^ Rainbow Reader, Fort Wayne, Indiana