The definition is contested as the answer has immense economic implications. If bamboo is a tree or timber, it belongs to the forest department and can be auctioned to the paper and pulp industry, often at throwaway rates.
Links:
[1] http://admin.indiaenvironmentportal.org.in/opinion/bamboo-tree-or-grass
[2] http://admin.indiaenvironmentportal.org.in/category/author/sunita-narain
[3] http://admin.indiaenvironmentportal.org.in/category/newspaper/down-earth
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-resources
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/information
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/bamboo
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/tribals
[8] http://admin.indiaenvironmentportal.org.in/category/thesaurus/minor-forest-produce
[9] http://admin.indiaenvironmentportal.org.in/category/thesaurus/maharashtra
[10] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-policy
[11] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-conservation
[12] http://admin.indiaenvironmentportal.org.in/category/thesaurus/benefit-sharing
[13] http://admin.indiaenvironmentportal.org.in/category/thesaurus/trees
[14] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-act-1927
[15] http://admin.indiaenvironmentportal.org.in/category/thesaurus/sunita-narain
[16] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forests
[17] http://admin.indiaenvironmentportal.org.in/category/thesaurus/gadchiroli-d
[18] http://admin.indiaenvironmentportal.org.in/category/thesaurus/forest-rights-act-2006
[19] http://admin.indiaenvironmentportal.org.in/category/thesaurus/india