Infant Mental Health is the developing capacity of the child from birth to age 3 to:
Experience, regulate, and express emotions
Form close and secure interpersonal relationships, and:
Explore the environment and learn
All in the context of family community, and cultural expectations for young children. Infant mental health is synonymous with healthy social and emotional development. Zero to Three Infant Mental Health Task Force, 2002
For more IMH definitions see:
Details of Zero to Three Definition:
http://www.zerotothree.org/site/PageServer?pagename=key_mental
Center for Early Education and Development at University of Minnesota
Tip Sheet for IMH Definition:
http://cehd.umn.edu/ceed/publications/tipsheets/attach3.pdf