''Octa'' means Eight, so a polygon which have 8 Angles, 8 Sides and 8 Vertices is known as Octagon.
Below figure represent, Octagon ABCDEFGH.
In the Above Figure 1 :
Octagon ABCDEFGH has
8 Sides  AB, BC, CD, DE, EF, FG, GH and HA
8 Angles  ABC, BCD, CDE, DEF, EFG, FGH, GHA and HAB
8 Vertices  A, B, C, D, E, F, G and H.
Below figure represent, Octagon PQRSTUVW
In the Above Figure 2 :
Octagon PQRSTUVW has
8 Sides  PQ, QR, RS, ST, TU, UV, VW and WP
8 Angles  PQR, QRS, RST, STU, TUV, UVW, VWP and WPQ
8 Vertices  P, Q, R, S, T, U, V and W

