Starbase 18 was the name of two different United Federation of Planets facilities active in the 23rd century.

The first was located on the asteroid 55 Hamilton, in the Hamilton star system. It was destroyed ca. 2252 with 87 Starfleet officers, civilians, and their families aboard. (TOS novel: The Children of Kings)

The second was located in the Alpha Arietis system in the 2270s. (TOS novel: The Wounded Sky)

This was the destination of USS Enterprise-A after departing Dianas in 2287. (Orion Press: The Dianasian Gift)

United Federation of Planets starbases
Starbases: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 9 - 10 - 11 - 12 - 14 - 15 - 16 - 17 - 18 - 20 - 21 - 22 - 23 - 24 - 25 - 26 - 27 - 28 - 29 - 30 - 31 - 32 - 33 - 36 - 37 - 38 - 39-Sierra - 41 - 42 - 43 - 44 - 45 - 47 - 47-Vanguard - 52 - 53 - 61 - 73 - 74 - 78 - 86 - 88 - 105 - 118 - 125 - 129 - 134 - 137 - 140 - 152 - 185 - 188 - 193 - 200 - 201 - 211 - 212 - 214 - 217 - 222 - 227 - 237 - 247 - 310 - 311 - 315 - 332 - 371 - 375 - 376 - 397 - 414 - 505 - 512 - 514 - 515 - 544 - 549 - 592 - 600 - 621 - 718 - 732 - 771 - 804 - 814 - 901 - Earhart - Expanse 3 - Expanse 4 - Expanse 7 - Expanse 12 - G-6 - Osiris - Pitcairn - Rockford - Unity 1 - Unity 2 - 001
Deep space stations: 1 - 2 - 3 - 4 - 5 - 6 - 7 - 9 - 9-II -10 - 12 - 14 - 17 - B-5 - K-1 - K-7 - K-12