What are the meanings of the word 'gender'?
According to the American Academy of Pediatrics the following is stated:

Sex is a biological construct, what is real:
Attributes that characterize biological maleness and femaleness including:
Chromosomes
Hormones
Anatomy

Gender is a social construct, or 'man-made':
Attitudes and feelings that a given culture associates with a persons biological sex such as:
Gender expression
Gender roles and behavioirs
Gender identity
This Varies by time and culture

I am not able to identify to what degree the AAP's views are accepted or rejected by other organizations.  

Source:
PDF, slide 7, from the AAP

Reference (Best attempt available for now)

Garofalo, R. (2013). Understanding Gender Nonconformity in Childhood and Adolescence
Webinar Video: https://www.aamc.org/initiatives/diversity/449804/aap2.html
Free Original Source PDF: https://www.aap.org/en-us/Documents/solgbt_webinar_transition_garofalo.pdf
Source PDF Bibliography [Free PDF]: http://www.pedsuniversity.org/Portals/2/biblios/Garofalo_Gender_biblio.pdf