POV-Ray has a number of reserved keywords which are listed below.
aa_level fog_offset reciprocal aa_threshold fog_type recursion_limit abs frequency red acos gif reflection acosh global_settings refraction adaptive glowing render adc_bailout gradient repeat agate granite rgb agate_turb gray_threshold rgbf all green rgbft alpha halo rgbt ambient height_field right ambient_light hexagon ripples angle hf_gray_16 rotate aperture hierarchy roughness arc_angle hollow samples area_light hypercomplex scale asc if scallop_wave asin ifdef scattering asinh iff seed assumed_gamma image_map shadowless atan incidence sin atan2 include sine_wave atanh int sinh atmosphere interpolate sky atmospheric_attenuation intersection sky_sphere attenuating inverse slice average ior slope_map background irid smooth bicubic_patch irid_wavelength smooth_triangle black_hole jitter sor blob julia_fractal specular blue lambda sphere blur_samples lathe spherical_mapping bounded_by leopard spiral box light_source spiral1 box_mapping linear spiral2 bozo linear_spline spotlight break linear_sweep spotted brick location sqr brick_size log sqrt brightness looks_like statistics brilliance look_at str bumps low_error_factor strcmp bumpy1 mandel strength bumpy2 map_type strlen bumpy3 marble strlwr bump_map material_map strupr bump_size matrix sturm camera max substr case max_intersections superellipsoid caustics max_iteration switch ceil max_trace_level sys checker max_value t chr merge tan clipped_by mesh tanh clock metallic test_camera_1 color min test_camera_2 color_map minimum_reuse test_camera_3 colour mod test_camera_4 colour_map mortar text component nearest_count texture composite no texture_map concat normal tga cone normal_map thickness confidence no_shadow threshold conic_sweep number_of_waves tightness constant object tile2 control0 octaves tiles control1 off torus cos offset track cosh omega transform count omnimax translate crackle on transmit crand once triangle cube onion triangle_wave cubic open true cubic_spline orthographic ttf cylinder panoramic turbulence cylindrical_mapping pattern1 turb_depth debug pattern2 type declare pattern3 u default perspective ultra_wide_angle degrees pgm union dents phase up difference phong use_color diffuse phong_size use_colour direction pi use_index disc pigment u_steps distance pigment_map v distance_maximum planar_mapping val div plane variance dust png vaxis_rotate dust_type point_at vcross eccentricity poly vdot else polygon version emitting pot vlength end pow vnormalize error ppm volume_object error_bound precision volume_rendered exp prism vol_with_light exponent pwr vrotate fade_distance quadratic_spline v_steps fade_power quadric warning falloff quartic warp falloff_angle quaternion water_level false quick_color waves file_exists quick_colour while filter quilted width finish radial wood fisheye radians wrinkles flatness radiosity x flip radius y floor rainbow yes focal_point ramp_wave z fog rand fog_alt range All reserved words are fully lower case. Therefore it is recommended that your identifiers contain at least one upper case character so it is sure to avoid conflict with reserved words. The following keywords are in the above list of reserved keywords but are not currently used by POV-Ray however they remain reserved.bumpy1 test_camera_1 bumpy2 test_camera_2 bumpy3 test_camera_3 incidence test_camera_4 pattern1 track pattern2 volume_object pattern3 volume_rendered spiral vol_with_light