La palabra del día: Alta

Feminine noun. This term has many meanings. Let’s see the ones that may be of interest to us.
1. Authorization given by the doctor for the reinstatement of a patient to ordinary life and also the documents that attest the discharge. We also call it el alta médica. The lexical locutions here are dar el alta (médica) de (el hospital/la clínica) and recibir el alta.
2. Act in which the taxpayer declares to the Treasury the exercise of industries or professions subject to tax. The lexical locution is darse de alta en and dar de alta en E.g.: Me di de alta como autónomo en la oficina de impuestos: I registered as a freelancer at the tax office; La oficina de impuestos me dio de alta en el sistema en enero de 2005: The tax office added me to the system in January 2005. In this case, the tax form to register in an industrial or professional activity is also called el alta.
3. Registration of a person in a service, professional body, association, etc., and the document that accredits such registration. The lexical locutions are also darse de alta en and dar de alta (en). E.g.: Si usted quiere darse de alta en el servicio puede hacerlo desde aquí: If you wish to register with the service, you can do so from here.
Note that the singular determinate and indeterminate articles we use with alta are always the masculine el and un: El alta, un alta. This is because of the combination of two reasons: 1. Alta is a feminine noun starting with a tónica (with the stress in the a); 2. In Spanish we do not have apostrophe. In the plural form the articles revert to the feminine form: las altas, unas altas. In amiga, we say la amiga because the stress is not in the first ‘a’ but in the ‘i’.
Remember, alta is always a feminine noun, even with the masculine article in front of it. E.g.: El alta médica. As far as I know, in Spanish we have just one word which is masculine in the singular and feminine in the plural; arte: el arte español del Siglo de Oro, las Bellas Artes.