A shield was an instrument of defense, carried in hand, and used to protect an individual by intercepting attacks from hand-held weapons, such as swords.
On the planet Taurus II in 2267, anthropoids living there had gigantic shields. Immediately after being shot by Spock wielding a phaser, one of the anthropoids threw its shield at Spock and a pair of accompanying officers under his command – Lieutenants Boma and Gaetano. (TOS: "The Galileo Seven")
According to the stage directions in the final draft script of "The Galileo Seven", this shield was made of leather and measured eleven foot. However, the same script also included an ultimately unused line of dialogue, in which Gaetano estimated that the shield "must be twelve feet high."
A year later, after Spock, Doctor McCoy, and Captain Kirk saved the life of Parmen during an away mission to Platonius, Kirk was awarded a shield originally carried by Pericles by the grateful Platonians. (TOS: "Plato's Stepchildren")
Shields were also used in defense when Kirk, Spock, and McCoy were forced to fight for a public spectacle later in 2268, around stardate 4041.2. (TOS: "Bread and Circuses")
Starfleet is known to have issued at least one type of shield to its forces in the late 23rd century. In 2287, the Enterprise was sent to the Planet of Galactic Peace, Nimbus III, to rescue ambassadors who had been taken hostage. The security forces were issued translucent riot shields that could be rolled up and stowed atop their backpacks. (Star Trek V: The Final Frontier)
The Bandi Groppler Zorn had three shields hanging as wall decorations in his office in Old Bandi City on Deneb IV in 2364. (TNG: "Encounter at Farpoint")
A targe was a type of Scottish shield, and Montgomery Scott used one as a wall hanging, along with other Scottish objects. (TOS: "By Any Other Name")